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The most important pathogenic species of the two genera, Staphylococcus aureus and Streptococcus pyogenes, have been studied more extensively than most other bacterial pathogens by microbiologists and physicians. In staphylococci, accessory genetic elements contribute to resistance to antibiotics, antiseptics, and disinfectants and are involved in genetic rearrangements and formation of virulence traits. Interestingly, the 3&apos; region of mecA in S. sciuri also has a high homology to the corresponding region of S. aureus. The presence of one or more copies of IS431 within mec and directed repeats at the ends of mecA, as well as open reading frames within mec that may encode recombinases, suggests that mecA and its associated DNA are mobile elements. The mechanism of maintenance of genetic diversity by natural transformation of S. pneumoniae which enables intraspecies and interspecies horizontal gene transfer differs fundamentally from that described for the mecA region in staphylococci. Staphylococcal phages are also involved in the expression of virulence determinants by mediating both positive and negative lysogenic conversion. Temperate phages of Streptococcus pyogenes are of particular interest because one class of major virulence factors, the streptococcal pyrogenic toxins, reside on their genomes. Of the virulence regulators, only the vir regulon occurs in all S. pyogenes strains, which highlights its exceptional role in pathogenicity. In contrast to S. pyogenes, S. pneumoniae does develop natural competence, and it has become known as the first microorganism with the capacity to take up free DNA experimentally from the environment.
Molecular organization of the mec region in S. aureus R155. Chromosomal sequences flanking this region are shown as hatched boxes. The mecA gene encodes the additional PBP2a. The mecA regulatory genes are designated mecRl and mecl. Tn554 is a transposon carrying ermA, encoding inducible MLS resistance. IS431 (the same as IS257) elements flank the kanamycin-tobramycin resistance plasmid pUBl 10. The directions of transcription for the mec genes are indicated by arrows.
Physical map of the 15.2-kb SaPIl region. Chromosomal sequences flanking this region are shown as hatched boxes, and the 17-bp direct repeats at left and right are shown as open boxes labelled att. The open reading frames encoding TSST-1 (tst), a homolog to VapE of Dichelobacter nodosus (vapE), a putative superantigen (ent), and an integrase (int) are shown as grey boxes. The directions of transcription for the genes are indicated by arrows.
Genetic organization of the ica operon and target sites for 1S256 insertions. (A) Overview of the ica operon and the adjacent icaR gene. The number and positions of IS256 insertions are marked by arrows. (B) Detailed representation of IS256 insertions detected in the icaC nucleotide sequence of S. epidermidis RPG2A and S. epidermidis 229 ( 131 ). IS256 target sites are underlined. Double lines mark identical target sequences identified in different insertional mutants. The designations above the targets indicate the individual names of the variants. Numbering of the nucleotide sequence corresponds to that of the published ica sequence (accession no. U43366).
mga virulence locus of S. pyogenes. Genes controlled by the Mga regulator (vir regulon) are indicated by arrows. The genes mga, mrp (M-related), emm, enn, scpA, and orfX form the core vir regulon. Genes are transcribed monocistronically from promoters P positively regulated by Mga. The interrupted line indicates unknown direct or indirect regulation of the genes by the Mga regulator, tt indicates transcriptional terminators, and at indicates attenuator sites. The genes encode the following products: mga, virulence gene regulator; mrp, M-related protein; emm, M protein; enn, IgA-binding protein; scpA, streptococcal C5a peptidase; sic, streptococcal inhibitor of complement; sof, serum opacity factor; scnA, streptococcic ropB, regulator of proteinase; speA, cysteine proteinase; oppA and oppB-F, oligopeptide peptidase; sagA, streptolysin S-associated gene (for details, see the text). Genes unaffected by Mga are indicated in the box.
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