Evolution and diversity of pyrimidine metabolism genes in lactic acid bacteria

Pyrimidine metabolism is an appropriate model for studying bacterial genome dynamics. Whereas pyrimidine de novo biosynthesis and salvage pathways are the same throughout the living world, the organization of the correspondin g pyr genes varies greatly from one organism to another. Lactobacillus plan tarum has a large pyr biosynthesis operon and a bicistronic cluster involve d in pyrimidine salvage. These features were the starling point of a compar ative study of all known pyr structures. All lactic acid bacteria possess t wo pyrD genes, both encoding a functional enzyme, except L, plantarum. Addi tionally L. plantarum has no pyrK gene, and its pyrP and upp genes form a c luster. pyr operonic structures are common among Gram(+) bacteria and are r elatively well conserved, but there is greater diversity amongst the lactic acid bacteria. This diversity could be due to adaptation to new ecological niches, correlated with functional gains or losses. This, in turn, could g ive rise to the differences in genome size.