A study on mutational dynamics of simple sequence repeats in relation to mismatch repair system in prokaryotic genomes.

Mutational bias toward expansion or contraction of simple sequence repeats (SSRs) is referred to as directionality of SSR evolution. In this communication, we report the mutational bias exhibited by mononucleotide SSRs occurring in the non-coding regions of several prokaryotic genomes. Our investigations revealed that the strains or species lacking mismatch repair (MMR) system generally show higher number of polymorphic SSRs than those species/strains having MMR system. An exception to this observation was seen in the mycobacterial genomes that are MMR deficient where only a few SSR tracts were seen with mutations. This low incidence of SSR mutations even in the MMR-deficient background could be attributed to the high fidelity of the DNA polymerases as a consequence of high generation time of the mycobacteria. MMR system-deficient species generally did not show any bias toward mononucleotide SSR expansions or contractions indicating a neutral evolution of SSRs in these species. The MMR-proficient species in which the observed mutations correspond to secondary mutations showed bias toward contraction of polymononucleotide tracts, perhaps, indicating low efficiency of MMR system to repair SSR-induced slippage errors on template strands. This bias toward deletion in the mononucleotide SSR tracts might be a probable reason behind scarcity for long poly A|T and G|C tracts in prokaryotic systems which are mostly MMR proficient. In conclusion, our study clearly demonstrates mutational dynamics of SSRs in relation to the presence/absence of MMR system in the prokaryotic system.