leishmania spp . lives in the gastrointestinal tract of the sand fly vector , and can be cultured using appropriate laboratory culture media as promastigotes . they can also exist in the vertebrate host macrophages in the amastigote form ( 13 ) . while asexual reproduction is known to occur in this species ( 4 ) , their sexual forms have not yet been discovered ( 5 ) . clonal reproduction is believed to occur among the protozoan parasites of the family trypanosomatidae ( 6 ) , considering that nuclear fusion occurs in some forms of this parasite that may give rise to sexual reproduction ( 7 , 8) . researchers have been unable to confirm sexual reproduction and identify sexual gametes of these microorganisms by using classical methods ( 9 ) . it should be noted that the exchange of genetic material in trypanosomatidae has been proven ( 1012 ) . leishmania is used as an intracellular molecular model for research in microbiology , immunology , and biochemistry ( 1 , 2 , 1320 ) . has 32,816,678 bp organized into 36 chromosomes ( 21 ) , with a total of 911 rna genes and 39 pseudo - genes ( 21 , 22 ) . producer protein genes are encoded as long polycistronic genes lacking transcription factors in l. major , trypanosoma brucei , and t. cruzi ( tritryp ) ( fig . 1 ) . the old world leishmania spp . has 36 chromosomes , while the new world leishmania spp . has 34 or 35 chromosomes . l. mexicana has linkage groups of chromosomes 8 and 29 as well as of chromosomes 30 and 36 , and l. braziliensis has a linkage group of chromosomes 20 and 34 ( 23 ) . the general pattern of nucleotide sequences of genes in 30 leishmania spp . organization of chromosomes of leishmania genes : clusters of genes on chromosomes 1 , 2 , 3 , 4 , and 35 are shown as thick lines . vertical lines indicate the right side of the chromosome 1 repeated sub - telomeric sequence . its genes have been organized into 2 converted polycistronic clusters , and mrna transcription is directed to the telomeres ( 2730 ) . chromosome 3 has about 79 genes , and is organized as 2 convergent polycistronic transcripts . they remain at the end of a gene that is transcribed in contrast to the previous clusters ( 28 , 29 , 31 ) . this mechanism differs from the other mechanisms , although they have a chromatin remodeling process ( 21 ) . in contrast to other members of trypanosomatidae , the leishmania genome does not have a sub - telomeric region ( species - specific genes ) and a transposable element . genome is organized in the nucleus , which contains chromosomal and episomal dna , and in the kinetoplasts , which comprise independently replicating dna molecules . the kinetoplasts have been separated and studied by ultracentrifugation , whereas the chromosomes have been studied by pulsed - field gel electrophoresis ( pfge ) . there are questions regarding the changes occurring in karyotype species , sexual reproduction in leishmania spp . , and the number of copies of each gene in each chromosome ( 33 ) . the electrophoretic patterns of leishmania spp . isolation by hybridization that parts of chromosomes can be common , but the genes hsp70 , hsp80 , adenylate cyclase , glyceraldehyde phosphate dehydrogenase , beta tubulin , phosphofructokinase , pho - sphoenolpyruvate carboxymethyl pyruvate kinase , pyruvate kinase , and ubiquitin are conserved . the chromosomes range from 400 to 900 kbp in size and contain mini - exons ( 5-spliced leader genes ) . chromosomal changes that occurred during the evolution of leishmania spp . have been confirmed , and the molecular karyotypes in the promastigote and amastigote forms have been found to be identical . ( 1 ) the l. major karyotype is completely conserved , even in different geographical regions , ( 2 ) the members of the l. braziliensis panamensis group have more than one karyotype , and ( 3 ) 
 l. mexicana amazonensis has highly diverse molecular karyotypes , even among those isolated from the same clinical samples . the mechanism of chromosomal polymorphism in leishmania spp . does not include removal or translocation . among the genes amplified in leishmania spp . this phenomenon of increasing the number of copies of genes involved in metabolic phenomena and environmental response appears to be important . regions of genes that are involved in drug resistance are increased by 220 folds in copy number . two genomic regions , namely , h - dna and r - dna ( encoding dihydrofolate reductase and thymidylate synthase ) , are chromosomal derivatives , which are surrounded by inverted repeats . the inverted repeats are involved in the supercoiling of amplified gene products ( 33 ) . methotrexate induces amplification of the r - dna and h - dna genomic regions . in a methotrexate - resistant l. tarentolae mutant , the h region the dihydrofolate reductase , thymidylate synthase , and ltdh genes in the h region are resistant to drugs ( 34 , 35 ) . gene amplification in the amphotericin b - resistant l. tarentolae occurs in the circular form in different chromosomes ( 36 ) . drug resistance to sodium stibogluconate ( pentostam ) in l. tarentolae is due to the amplification of a gene described by haimeur and ouellett , which encodes a 770-amino acid - long protein ( 37 ) . gene expression control among the members of the parasitic trypanosomatidae family involves unusual antigenic shifts , involving dna rearrangements , generation of polycistronic transcripts from multi - copy genes , and post - transcriptional modification by trans - splicing and rna editing ( 38 ) . the genetic information of most organisms has been discovered in cdna sequences known as expressed sequence tags ( est ) ( 39 ) . it should be noted that most leishmania genes have no introns ( 40 ) , and that chromosomal dna is used as the template for cloning by pcr ( 4145 ) . gene transcription to produce proteins in eukaryotes involves rna polymerase ii and transcription by rna polymerase i to produce ribosomal rna . in kinetoplastids , gene transcription involves rna polymerase i and a trans - splicing mechanism ( 46 ) . this sequence is encoded by a gene duplication cluster , 1.35 kb in length , which is known as a mini - exon or a trans - splice . mini - exon mrna was first identified as being related to the trypanosome variable surface glycoprotein ( 47 ) . analyzed the sequence of leishmania chromosome 1 , which is the smallest of all the chromosomes . a total of 39 genes were transcribed from a strand of dna and 50 other genes in a polycistronic transcript ( 48 ) . also analyzed leishmania chromosome 27 and indicated that the organization of transcription of leishmania genes is a complex process . non - coding rnas , about 300600 nucleotides long , are known to be expressed only in the amastigotes ; these rnas are transcribed by rna polymerase ii . both sense and antisense transcripts are processed by trans - splicing and polyadenylation , but the antisense transcripts are transcribed 10 folds lesser than the sense transcripts . it should be noted that these molecules are not transcribed in promastigotes , and that rna stability in promastigotes is less than in amastigotes ( 50 ) . there is a 35-nucleotide - long sequence known as a spliced leader ( sl ) or 5-mini exon at the 5 end of leishmania mrna transcripts . the sl sequence is at the 5-end of a preliminary transcript about 85 nucleotides in length that contains a 5-exon - intron connection adjacent to the 3-spliced leader ( fig . 2 and 3 ) . comparison of cis- and trans - splicing : in cis - splicing , pair bases u1 small nuclear ribonucleoprotein ( snrnp ) are in the 5 sl [ ? ] and u2 snrnps are in the break point , while intron breaks two exons are connected . in trans - splicing , a 5-splice site on the mrna for binding to u1 snrnp is absent . instead , a 5-splice site produced by the donor sl snrnp interacts with u2 in the 3-splice site . cis - splicing and trans - splicing : there are 4 exons in the initial transcript , which contains both exons and introns . in the cis - splicing phenomenon , the 3 introns are removed , and the exons are connected . in trans - trans - splicing , there is a 50-nucleotide - long interval at the 3-end of sl , which is known as the sl intron sequence ( slis ) . density centrifugation analyses have shown that sl mrna is in the 60s rrna , but slis is in the 40s rrna . it is likely that the observed nucleoprotein particles are the same spliceosomes that can be observed in other microorganisms ( 51 , 52 ) . since the discovery of trans - splicing in leishmania spp . trans - splicing is an essential stage of eukaryotic precursor mrna and is not observed in mammals , insects , yeast , and plants ( 54 ) . this phenomenon is observed in rotifera ( 55 ) , dinoflagellates ( 56 ) , nematodes , and protozoan parasites ( 5760 ) as shown in fig . trans - splicing in metazoan parasites : a ) transcription occurs via a polycistronic transcript and trans - splicing . b ) the phenomenon of transcription and trans - splicing in metazoan genes ( worms ) . comparison of the characteristics of the genomes of 3 species of leishmania 
 ( 22 ) 
  the organization and regulation of gene expression in trypanosomatid parasites differs from that of other cells . the genes in the parasites of family trypanosomatidae are organized as long polycistronic transcripts ( more than 100300 kb ) on the same dna strand . the genes encoding proteins are transcribed from unknown promoters , and precursor polycistronic rna is produced . the trans - splicing mechanism includes a mini - exon containing 39 nucleotides , which is not translated . the ag is at the 3 of the splice acceptor site downstream of a polypyrimidine tract . there are no introduced polyadenylation signals in kinetoplastidae undefined and instead choose to place poly a site depends on positions upstream acceptor site ( 61 . ) gopta et al . analyzed chromosomes 1 and 3 of l. major and predicted the positions of trans - splicing with 92% accuracy . the following components are present : ( 1 ) nucleotide a , ( 2 ) a polypyrimidine rich stretch of t and c , varying in size from 5 to 100 nucleotides with purine bases occasionally located between the t and c , ( 3 ) a variable spacer , and ( 4 ) a 3-acceptor site consisting of ag ( 62 ) . . generates pyrimidine nucleic acids via de novo biosynthesis , but obtains purine nucleic acids via a salvage process ( 6366 ) . this study indicated that salvage activities in the amastigote stage would limit the effectiveness of chemotherapy in patients infected with leishmania spp . salvage activities do not involve rna transcripts and likely occur via posttranscriptional modifications ( 67 ) . if dna replication is inhibited in the kinetoplast by ethidium bromide , energy production will reduced in the parasite ( 70 ) . the extra - chromosomal dna is located in the kinetoplast organelle in an arrangement similar to that in the mitochondria of the eukaryotes . the kinetoplast has a particular dna topology , which is not found in other eukaryotic cells ( fig . 5 ) , and is composed of large circular molecules up to about 50,000 nucleotides that are known as maxicircles . they are not present in large numbers . these circular molecules carry the genes encoding the enzymes and coenzymes involved in the krebs cycle ( 7174 ) . structure of the kinetoplast disk and the proteins involved in its replication sse1 , structure -specific endonuclease 1 ; umsbp , universal minicircle sequence - binding protein ( http://www.pnas.org/content/101/13/4333/f2.expansion.html ) other dna molecules that are present in larger numbers but have fewer nucleotides ( 6002,500 bp ) are known as minicircles . chritidia fasciculate has 25 maxicircles ( each containing 37,000 nucleotides ) and 5,000 minicircles ( each containing 2,500 nucleotides ) ( 7274 ) , some of which have been identified as free - form molecules ( 73 ) . a minicircle dna sequence is conserved among all members of the family kinetoplastidae ( 75 ) , and some believe that this sequence is the origin of replication of the minicircle ( 76 ) . large and small circles can exist inside each other so that each loop intercalates with 2 other minicircle loops and eventually  maxicircles and minicircles  become intertwined with each other ( interlocked or catenated ) and a heavy molecule ( about 400s ) is formed during the extraction of parasite dna that is distinct from the chromosomal dna sediment . when minicircles replicate , some are released ( fig . when replication is completed , a replicated minicircle will become connected to the kinetoplast ( 75 , 77 , 78 ) . the blank section of the kinetoplast is restored by dna topoisomerase ii ( 79 ) . 
 in vivo replication of a kinetoplast shown as a disk section with catenated minicircles surrounded by dna polymerase beta and dna topoisomerase ii . two newly synthesized minicircles are shown in bold ( http://www.jbc.org/content/272/33/20787.full.pdf+html ) the origin of replication of the kinetoplast is recognized by a protein known as umsbp ( universal minicircle sequence - binding protein ) . this reaction is regulated in vivo by an oxidation - reduction reaction ( 80 , 81 ) . the inhibition of umsbp halts the growth of the parasite ( 82 ) . a zinc ion ( zn ) is involved in this process , and is essential for connecting umsbp to dna ( 83 ) . it should be noted that replication of the members of kinetoplastidae occurs via different mechanisms ( 84 ) . the functions of minicircles were not clarified until recently , and the genes of some of the enzymes involved in krebs cycle were not observed . the discovery of the rna editing phenomenon was an exciting new finding ( 8587 ) . it was found that the parasites have copies of rna molecules , which are altered because of posttranscriptional modification . this is accompanied by deletion or insertion of a number of nucleotide residues ( mostly uracil ) . rna editing emits signals by guide rna - derived transcripts of minicircles ( 88 , 89 ) or maxicircles ( 90 ) ( fig . 7 ) . a grna - binding complex is involved in the processing of a grna , which includes polyadenylation and stabilization of the edited mrna transcript ( 89 ) . the kinetoplast of l. tarentolae has a 9s rrna ( 9194 ) and a 12s rrna ( 9295 ) , but does not have supercoiled circles ( 96 ) . rna editing of cytochrome oxidase b of leishmania tarantula ( http://dna.kdna.ucla.edu/trypanosome/index.html ) the rna editing phenomenon produces deletions , replacements , and insertions in mrna transcripts ( 86 , 87 , 97 ) . an edited transcript mrna has important effects , and sometimes half of the nucleotides are altered ( fig . it should be noted that although the changes may be small , its effect is important . for example , replacement of a c nucleotide by u in the human apolipoprotein b transcript ( fig . model rna editing in the kinetoplast : addition of u ( left ) , removal of u ( center ) or formation of a chimera ( right ) in an mrna transcript are performed by tutase ( http://dna.kdna.ucla.edu/trypanosome/images/kablea.jpg ) rna editing in human apolipoprotein b the tga codon ( stop codon ) of the leishmania spp . when the parasite glycosomal cycle is reduced , the mitochondrial ( kinetoplast ) volume is increased , and vice versa ( 99 ) . the characteristics of kinetoplast dna has led to its choice as a target for drug therapy ( 99 , 100 ) . the haploid genome of leishmania spp . has 32,816,678 bp organized into 36 chromosomes ( 21 ) , with a total of 911 rna genes and 39 pseudo - genes ( 21 , 22 ) . producer protein genes are encoded as long polycistronic genes lacking transcription factors in l. major , trypanosoma brucei , and t. cruzi ( tritryp ) ( fig . 1 ) . the old world leishmania spp . has 36 chromosomes , while the new world leishmania spp . l. mexicana has linkage groups of chromosomes 8 and 29 as well as of chromosomes 30 and 36 , and l. braziliensis has a linkage group of chromosomes 20 and 34 ( 23 ) . the general pattern of nucleotide sequences of genes in 30 leishmania spp . organization of chromosomes of leishmania genes : clusters of genes on chromosomes 1 , 2 , 3 , 4 , and 35 are shown as thick lines . the direction of mrna transcription is indicated . vertical lines indicate the right side of the chromosome 1 repeated sub - telomeric sequence . its genes have been organized into 2 converted polycistronic clusters , and mrna transcription is directed to the telomeres ( 2730 ) . chromosome 3 has about 79 genes , and is organized as 2 convergent polycistronic transcripts . they remain at the end of a gene that is transcribed in contrast to the previous clusters ( 28 , 29 , 31 ) . this mechanism differs from the other mechanisms , although they have a chromatin remodeling process ( 21 ) . in contrast to other members of trypanosomatidae , the leishmania genome does not have a sub - telomeric region ( species - specific genes ) and a transposable element . genome is organized in the nucleus , which contains chromosomal and episomal dna , and in the kinetoplasts , which comprise independently replicating dna molecules . the kinetoplasts have been separated and studied by ultracentrifugation , whereas the chromosomes have been studied by pulsed - field gel electrophoresis ( pfge ) . there are questions regarding the changes occurring in karyotype species , sexual reproduction in leishmania spp . , and the number of copies of each gene in each chromosome ( 33 ) . the electrophoretic patterns of leishmania spp . , hsp80 , adenylate cyclase , glyceraldehyde phosphate dehydrogenase , beta tubulin , phosphofructokinase , pho - sphoenolpyruvate carboxymethyl pyruvate kinase , pyruvate kinase , and ubiquitin are conserved . the chromosomes range from 400 to 900 kbp in size and contain mini - exons ( 5-spliced leader genes ) . chromosomal changes that occurred during the evolution of leishmania spp . have been confirmed , and the molecular karyotypes in the promastigote and amastigote forms have been found to be identical . ( 1 ) the l. major karyotype is completely conserved , even in different geographical regions , ( 2 ) the members of the l. braziliensis panamensis group have more than one karyotype , and ( 3 ) 
 l. mexicana amazonensis has highly diverse molecular karyotypes , even among those isolated from the same clinical samples . the mechanism of chromosomal polymorphism in leishmania spp . does not include removal or translocation . among the genes amplified in leishmania spp . this phenomenon of increasing the number of copies of genes involved in metabolic phenomena and environmental response appears to be important . regions of genes that are involved in drug resistance are increased by 220 folds in copy number . two genomic regions , namely , h - dna and r - dna ( encoding dihydrofolate reductase and thymidylate synthase ) , are chromosomal derivatives , which are surrounded by inverted repeats . the inverted repeats are involved in the supercoiling of amplified gene products ( 33 ) . methotrexate induces amplification of the r - dna and h - dna genomic regions . in a methotrexate - resistant l. tarentolae mutant , the h region the dihydrofolate reductase , thymidylate synthase , and ltdh genes in the h region are resistant to drugs ( 34 , 35 ) . gene amplification in the amphotericin b - resistant l. tarentolae occurs in the circular form in different chromosomes ( 36 ) . drug resistance to sodium stibogluconate ( pentostam ) in l. tarentolae is due to the amplification of a gene described by haimeur and ouellett , which encodes a 770-amino acid - long protein ( 37 ) . gene expression control among the members of the parasitic trypanosomatidae family involves unusual antigenic shifts , involving dna rearrangements , generation of polycistronic transcripts from multi - copy genes , and post - transcriptional modification by trans - splicing and rna editing ( 38 ) . the genetic information of most organisms has been discovered in cdna sequences known as expressed sequence tags ( est ) ( 39 ) . it should be noted that most leishmania genes have no introns ( 40 ) , and that chromosomal dna is used as the template for cloning by pcr ( 4145 ) . gene transcription to produce proteins in eukaryotes involves rna polymerase ii and transcription by rna polymerase i to produce ribosomal rna . in kinetoplastids , gene transcription involves rna polymerase i and a trans - splicing mechanism ( 46 ) . this sequence is encoded by a gene duplication cluster , 1.35 kb in length , which is known as a mini - exon or a trans - splice . mini - exon mrna was first identified as being related to the trypanosome variable surface glycoprotein ( 47 ) . analyzed the sequence of leishmania chromosome 1 , which is the smallest of all the chromosomes . a total of 39 genes were transcribed from a strand of dna and 50 other genes in a polycistronic transcript ( 48 ) . also analyzed leishmania chromosome 27 and indicated that the organization of transcription of leishmania genes is a complex process . non - coding rnas , about 300600 nucleotides long , are known to be expressed only in the amastigotes ; these rnas are transcribed by rna polymerase ii . both sense and antisense transcripts are processed by trans - splicing and polyadenylation , but it should be noted that these molecules are not transcribed in promastigotes , and that rna stability in promastigotes is less than in amastigotes ( 50 ) . there is a 35-nucleotide - long sequence known as a spliced leader ( sl ) or 5-mini exon at the 5 end of leishmania mrna transcripts . the sl sequence is at the 5-end of a preliminary transcript about 85 nucleotides in length that contains a 5-exon - intron connection adjacent to the 3-spliced leader ( fig . 2 and 3 ) . comparison of cis- and trans - splicing : in cis - splicing , pair bases u1 small nuclear ribonucleoprotein ( snrnp ) are in the 5 sl [ ? ] and u2 snrnps are in the break point , while intron breaks two exons are connected . in trans - splicing , a 5-splice site on the mrna for binding to u1 snrnp is absent . instead , a 5-splice site produced by the donor sl snrnp interacts with u2 in the 3-splice site . cis - splicing and trans - splicing : there are 4 exons in the initial transcript , which contains both exons and introns . in the cis - splicing phenomenon , the mrna contains 4 exons and 3 introns . the 3 introns are removed , and the exons are connected . in trans - trans - splicing , there is a 50-nucleotide - long interval at the 3-end of sl , which is known as the sl intron sequence ( slis ) . density centrifugation analyses have shown that sl mrna is in the 60s rrna , but slis is in the 40s rrna . it is likely that the observed nucleoprotein particles are the same spliceosomes that can be observed in other microorganisms ( 51 , 52 ) . since the discovery of trans - splicing in leishmania spp . trans - splicing is an essential stage of eukaryotic precursor mrna and is not observed in mammals , insects , yeast , and plants ( 54 ) . this phenomenon is observed in rotifera ( 55 ) , dinoflagellates ( 56 ) , nematodes , and protozoan parasites ( 5760 ) as shown in fig . trans - splicing in metazoan parasites : a ) transcription occurs via a polycistronic transcript and trans - splicing . b ) the phenomenon of transcription and trans - splicing in metazoan genes ( worms ) . comparison of the characteristics of the genomes of 3 species of leishmania 
 ( 22 ) 
  the organization and regulation of gene expression in trypanosomatid parasites differs from that of other cells . the genes in the parasites of family trypanosomatidae are organized as long polycistronic transcripts ( more than 100300 kb ) on the same dna strand . the genes encoding proteins are transcribed from unknown promoters , and precursor polycistronic rna is produced . the trans - splicing mechanism includes a mini - exon containing 39 nucleotides , which is not translated . the ag is at the 3 of the splice acceptor site downstream of a polypyrimidine tract . polyadenylation in leishmania spp . requires trans - splicing and differs from that of other eukaryotes . there are no introduced polyadenylation signals in kinetoplastidae undefined and instead choose to place poly a site depends on positions upstream acceptor site ( 61 . ) analyzed chromosomes 1 and 3 of l. major and predicted the positions of trans - splicing with 92% accuracy . the following components are present : ( 1 ) nucleotide a , ( 2 ) a polypyrimidine rich stretch of t and c , varying in size from 5 to 100 nucleotides with purine bases occasionally located between the t and c , ( 3 ) a variable spacer , and ( 4 ) a 3-acceptor site consisting of ag ( 62 ) . leishmania spp . generates pyrimidine nucleic acids via de novo biosynthesis , but obtains purine nucleic acids via a salvage process ( 6366 ) . this study indicated that salvage activities in the amastigote stage would limit the effectiveness of chemotherapy in patients infected with leishmania spp . salvage activities do not involve rna transcripts and likely occur via posttranscriptional modifications ( 67 ) . if dna replication is inhibited in the kinetoplast by ethidium bromide , energy production will reduced in the parasite ( 70 ) . the extra - chromosomal dna is located in the kinetoplast organelle in an arrangement similar to that in the mitochondria of the eukaryotes . the kinetoplast has a particular dna topology , which is not found in other eukaryotic cells ( fig . 5 ) , and is composed of large circular molecules up to about 50,000 nucleotides that are known as maxicircles . they are not present in large numbers . these circular molecules carry the genes encoding the enzymes and coenzymes involved in the krebs cycle ( 7174 ) . structure of the kinetoplast disk and the proteins involved in its replication sse1 , structure -specific endonuclease 1 ; umsbp , universal minicircle sequence - binding protein ( http://www.pnas.org/content/101/13/4333/f2.expansion.html ) other dna molecules that are present in larger numbers but have fewer nucleotides ( 6002,500 bp ) are known as minicircles . chritidia fasciculate has 25 maxicircles ( each containing 37,000 nucleotides ) and 5,000 minicircles ( each containing 2,500 nucleotides ) ( 7274 ) , some of which have been identified as free - form molecules ( 73 ) . a minicircle dna sequence is conserved among all members of the family kinetoplastidae ( 75 ) , and some believe that this sequence is the origin of replication of the minicircle ( 76 ) . large and small circles can exist inside each other so that each loop intercalates with 2 other minicircle loops and eventually  maxicircles and minicircles  become intertwined with each other ( interlocked or catenated ) and a heavy molecule ( about 400s ) is formed during the extraction of parasite dna that is distinct from the chromosomal dna sediment . when replication is completed , a replicated minicircle will become connected to the kinetoplast ( 75 , 77 , 78 ) . the blank section of the kinetoplast is restored by dna topoisomerase ii ( 79 ) . 
 in vivo replication of a kinetoplast shown as a disk section with catenated minicircles surrounded by dna polymerase beta and dna topoisomerase ii . two newly synthesized minicircles are shown in bold ( http://www.jbc.org/content/272/33/20787.full.pdf+html ) the origin of replication of the kinetoplast is recognized by a protein known as umsbp ( universal minicircle sequence - binding protein ) . this reaction is regulated in vivo by an oxidation - reduction reaction ( 80 , 81 ) . the inhibition of umsbp halts the growth of the parasite ( 82 ) . a zinc ion ( zn ) is involved in this process , and is essential for connecting umsbp to dna ( 83 ) . it should be noted that replication of the members of kinetoplastidae occurs via different mechanisms ( 84 ) . the functions of minicircles were not clarified until recently , and the genes of some of the enzymes involved in krebs cycle were not observed . the discovery of the rna editing phenomenon was an exciting new finding ( 8587 ) . it was found that the parasites have copies of rna molecules , which are altered because of posttranscriptional modification . this is accompanied by deletion or insertion of a number of nucleotide residues ( mostly uracil ) . rna editing emits signals by guide rna - derived transcripts of minicircles ( 88 , 89 ) or maxicircles ( 90 ) ( fig . 7 ) . a grna - binding complex is involved in the processing of a grna , which includes polyadenylation and stabilization of the edited mrna transcript ( 89 ) . the kinetoplast of l. tarentolae has a 9s rrna ( 9194 ) and a 12s rrna ( 9295 ) , but does not have supercoiled circles ( 96 ) . rna editing of cytochrome oxidase b of leishmania tarantula ( http://dna.kdna.ucla.edu/trypanosome/index.html ) the rna editing phenomenon produces deletions , replacements , and insertions in mrna transcripts ( 86 , 87 , 97 ) . an edited transcript mrna has important effects , and sometimes half of the nucleotides are altered ( fig . it should be noted that although the changes may be small , its effect is important . for example , replacement of a c nucleotide by u in the human apolipoprotein b transcript ( fig . model rna editing in the kinetoplast : addition of u ( left ) , removal of u ( center ) or formation of a chimera ( right ) in an mrna transcript are performed by tutase ( http://dna.kdna.ucla.edu/trypanosome/images/kablea.jpg ) rna editing in human apolipoprotein b the tga codon ( stop codon ) of the leishmania spp . when the parasite glycosomal cycle is reduced , the mitochondrial ( kinetoplast ) volume is increased , and vice versa ( 99 ) . the characteristics of kinetoplast dna has led to its choice as a target for drug therapy ( 99 , 100 ) . leishmania is a protozoan parasite with some similarities and differences as compared to other eukaryotic cells . it shares some characteristics with prokaryotic cells , such as polycistronic transcription ( 31 , 38 ) . researchers have been attracted to its unique characteristics . in recent years , leishmania spp . has been used as a host for production of recombinant proteins . an appropriate host is an important factor in production of recombinant proteins ( drugs ) . prokaryotes such as escherichia coli need simple and inexpensive culture media and have a short proliferation time . however , prokaryotes do not generate posttranslational modifications such as gly - cosylation , phosphorylation , and car - boxylation . some eukaryotic proteins are non - functional after translation in e. coli , and some of them become aggregated as inclusion bodies in the host cell cytoplasm . replication of yeasts such as pichia pastoris , saccharomyces cerevisiae , and schizosaccharomyces pombe also requires significant culture time , and posttranslational modifications are not perfect processes . other types of eukaryotic cell cultures tend to be expensive and require specialized culture conditions and laboratory equipment . because leishmania spp . is maintained easily in nnn culture medium at low cost and can multiply quickly , it is preferred over other species . this makes leishmania spp . a suitable host for production of recombinant protein drugs ( 101104 ) . efforts undertaken thus far have allowed the production of some therapeutic proteins ( 105108 ) . however , more research is needed before it can be used extensively as a host for the production of recombinant proteins . researchers in biochemistry , pharmacology , and immunology , who are engaged in new drug development as well as production and testing vaccines , need appropriate cell models . is an appropriate model for testing such enzyme inhibitors to investigate the progression of anti - parasitic and anti - cancer drugs ( 111 , 112 ) . small ubiquitin - like modifier protein ( sumo ) is a fusion protein that can be added as a reversible tag to n terminal recombinant proteins ( eukaryotes and prokaryotes ) to provide stability and solubility to proteins ( 113 , 114 ) . sumo is produced by leishmania spp . and can be used as a stable and soluble factor for the production of recombinant proteins in leishmania promastigote ( 115 ) . one disadvantage is that non - coding rnas act as mrna stability factors are not transcribed in the leishmania promastigote . because mrna is not stable in promastigotes ( 50 ) , this form of leishmania spp . biosynthesis de novo : the de novo purine biosynthetic pathway produces purines which represent the building blocks for dna and rna synthesis 
 cistron : a segment dna equivalent to gene for function ( protein or enzymes ) 
 diploid ; an organism with sexual cycle is diploid and has one chromosome set from each of its parents 
 haploid : the haploid means usual number of chromosomes set in somatic cells of common organisms . organisms that have not sexual cycle are haploid 
 inversion : chromosome break of the two areas separated pieces is back by reversal from chromosome breakage 
 inverted repeats : is a sequence of nucleotides that is the reversed complement of another sequence further downstream 
 poly cistronic : there are some cistrons on one mrna 
 polyploidy : increase in chromosome set number 
 pseudo - genes : are copy of original gene sequence , but lacked the necessary sequences for function . these genes from genetically similar to functional genes , but they have containing multiple mutations 
 snrnp : small nuclear ribonucleoproteins , are rna - protein complexes , they will combined with unmodified pre - mrna and various other proteins to spliceosome formation . sub telomeric : sub telomeric is a region near the end of chromosomes composed of polymorphic repetitive dna .
leishmania is a protozoan parasite belonging to the family trypanosomatidae , which is found among 88 different countries . 
 the parasite lives as an amastigote in vertebrate macrophages and as a promastigote in the digestive tract of sand fly . 
 it can be cultured in the laboratory using appropriate culture media . 
 although the sexual cycle of leishmania has not been observed during the promastigote and amastigote stages , it has been reported by some researchers . 
 leishmania has eukaryotic cell organization . 
 cell culture is convenient and cost effective , and because posttranslational modifications are common processes in the cultured cells , the cells are used as hosts for preparing eukaryotic recombinant proteins for research . 
 several transcripts of rdna in the leishmania genome are suitable regions for conducting gene transfer . 
 old world leishmania spp . has 36 chromosomes , while new world leishmania spp . 
 has 34 or 35 chromosomes . 
 the genomic organization and parasitic characteristics have been investigated . 
 leishmania spp . has a unique genomic organization among eukaryotes ; the genes do not have introns , and the chromosomes are smaller with larger numbers of genes confined to a smaller space within the nucleus . 
 leishmania spp . 
 genes are organized on one or both dna strands and are transcribed as polycistronic ( prokaryotic - like ) transcripts from undefined promoters . 
 regulation of gene expression in the members of trypanosomatidae differs from that in other eukaryotes . 
 the trans - splicing phenomenon is a necessary step for mrna processing in lower eukaryotes and is observed in leishmania spp . 
 another particular feature of rna editing in leishmania spp . 
 is that mitochondrial genes encoding respiratory enzymes are edited and transcribed . 
 this review will discuss the chromosomal and mitochondrial ( kinetoplast ) genomes of leishmania spp . as well as the phenomenon of rna editing in the kinetoplast genome .