Life on Arginine for Mycoplasma hominis: Clues from Its Minimal Genome and Comparison with Other Human Urogenital Mycoplasmas
2009
Mycoplasma hominisis an opportunistic human
mycoplasma. Two other pathogenic human species, M. genitalium and
Ureaplasma parvum, reside within the same natural niche as M. hominis: the urogenital tract. These three species have overlapping, but distinct, pathogenic roles. They have
minimal genomesand, thus, reduced metabolic capabilities characterized by distinct energy-generating
pathways.
Analysisof the M. hominis PG21 genome sequence revealed that it is the second smallest genome among
self-replicatingfree living organisms (665,445 bp, 537 coding sequences (CDSs)). Five clusters of genes were predicted to have undergone
horizontal gene transfer(HGT) between M. hominis and the phylogenetically distant U. parvum species. We reconstructed M. hominis metabolic pathways from the predicted genes, with particular emphasis on energy-generating pathways. The Embden–Meyerhoff–Parnas pathway was incomplete, with a single enzyme absent. We identified the three proteins constituting the
arginine dihydrolasepathway. This pathway was found essential to promote growth in vivo. The predicted presence of
dimethylarginine dimethylaminohydrolasesuggested that arginine catabolism is more complex than initially described. This enzyme may have been acquired by HGT from non-
mollicutebacteria. Comparison of the three minimal
mollicutegenomes showed that 247 CDSs were common to all three genomes, whereas 220 CDSs were specific to M. hominis, 172 CDSs were specific to M. genitalium, and 280 CDSs were specific to U. parvum. Within these species-specific genes, two major sets of genes could be identified: one including genes involved in various energy-generating pathways, depending on the energy source used (glucose, urea, or arginine) and another involved in cytadherence and virulence. Therefore, a minimal
mycoplasmacell, not including cytadherence and virulence-related genes, could be envisaged containing a core genome (247 genes), plus a set of genes required for providing energy. For M. hominis, this set would include 247+9 genes, resulting in a theoretical
minimal genomeof 256 genes.
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