BMC Genomics 2005, 6:136 http://www.biomedcentral.com/1471-2164/6/136 FAigpuyrriemi1dine-purine walk of human (+ strand) mtDNA A pyrimidine-purine walk of human (+ strand) mtDNA. Genes having mRNA synonymous with the (+) strand or (-) strand are indicated by color and also shown on the strand bars below the graph. An inset of a tRNA-containing section of the graph around the origin of light strand replication (OL) is shown. phorylation membrane-complex transcripts on the (+) strand were purine-rich, just like the rest of its genome. To place the extreme purine richness of Dictyostelium in con- text, the human mtDNA genome (+) strand is 44 % purine while that of the plant Arabidopsis thaliana is 50 % purine. The purine abundance of the mitochondrial genome of other species is listed in additional file 1: Table S1. Slime molds contain the most purine-rich (+) strand of any of the 23 organisms we examined, while mammals, birds, and a green algae species, Pedinomonas minor, contain the least amount of purine (44%). We analyzed the pyrimidine and purine content of mito- chondrial transcripts from many diverse eukaryotic spe- cies (Table 1). Unlike vertebrate mtDNA that lacks genes for soluble proteins, plant, fungi, and protist mtDNA encode genes for many ribosomal proteins and a few other soluble proteins. From this data we defined the fol- lowing three rules that apply to the pyrimidine-purine richness of mitochondrial transcripts. Rule 1) Oxidative phosphorylation complex and other membrane protein transcripts are pyrimidine-rich. Rule 2) Ribosomal RNA is purine-rich. Rule 3) Soluble protein transcripts are purine-rich. Table 1 lists the number of genes in each species that fol- low each rule, along with the number that fail. There were Page 3 of 12 (page number not for citation purposes)