Effects of genomic context on Escherichia coli DNA mutation rate

MARTINA MA

San Miguel de Tucumán

Congreso; XLV Reunión Anual de SAIB; 2009

In Escherichia coli the transitory hemimethylated state of adenines in GATC sequences provide the strand discrimination signal to direct the DNA mismatch repair system (MRS). Strains deficient in any of the principal components of the MRS (MutS, -L, -H, and Dam proteins) are mutators. It has been reported using plasmid heteroduplexes that a decrease in the number of GATC sequences within these vectors lowered the efficiency of mismatch repair in vitro and in vivo. We analyzed the effect of genomic GATC density on mutation rate in E. coli. We introduced a mutated copy of a gene able to confer antibiotic resistance within a genome region with a high or low density of GATC sequences and analyzed the reversion rate of the mutation. Our results show, unexpectedly, that the reversion rate was lower in strains containing the mutated gene in a genomic region with a low density of GATC sequences, than those located in a high density context. Moreover, the reversion rate of the mutated gene located in the low GATC density region of a MRS deficient strain was lower than the reversion rate of the mutated gene located in the GATC high density region of a wild type strain. Two possibilities are discussed: 1- Other(s) cis or trans acting factor(s) influence the mutation rate; 2- The high and low GATC density genomic regions differently affect the transcription rate of the reporter gene.