Abstract

DNA repair processes are crucial in maintaining genome integrity. Postreplication repair systems target specific misincorporations which occur during DNA replication. The mismatch repair system corrects most base pair mismatches, the nucleotide excision repair system corrects bulky lesions and the very short patch repair system of E. coli , repairs G-T mismatches that occur in the CCWGG sequence context. The purpose of this study is to investigate the roles of very short patch repair, mismatch repair and nucleotide excision repair systems, and their interactions in the presence of mutagen induced lesions caused by 2-aminopurine and 5-azacytidine. We have examined the regulation of the dcm and vsr genes of the very short patch repair system, and have found that Vsr is growth phase regulated while Dcm levels remain constant throughout the various stages of the cell's growth. The Vsr regulation was also found to be post-transcriptional and also, although to a lesser extent, operon structure regulated. This helps explain why 5-methylcytosines are hotspots for mutations in E. coli . We have shown that Dcm, the only cytosine methyltransferase in E. coli , when bound to its target site CCWGG and in the presence of mutagens, 2-aminopurine and 5-azacytidine, affects DNA repair and replication, respectively. Our results indicate that the 2-aminopurine effect is due to increased mispairing of 5-methycytosines with 2-aminopurine compared to normal cytosine. 5-azacytidine stimulates C-to-G mutations which are independent of the methylase. We have evidence which suggests that the lesions leading to the C-to-G mutations are C-C mismatches. We also have results which indicate that these 5-azacytidine induced lesions are repaired by the nucleotide repair system as well as the MutS and MutL proteins of the mismatch repair system. Other lesions caused by the drug include a methylase dependent lesion. Dcm induced lesions occur when the cytosine methyltransferase binds covalently to 5-azacytosine containing DNA. Our results show that this lesion leads to a reduction in DNA replication, cell growth and cell division.