MECHANISM OF EXPRESSION OF DNA-REPAIR GENE VSR, AN ESCHERICHIA-COLI GENE THAT OVERLAPS THE DNA CYTOSINE METHYLASE GENE, DCM

The DNA cytosine methylase gene of Escherichia coli, dcm, overlaps an open reading frame (ORF) that continues in +1 register past the end of dcm. This ORF codes for a gene, vsr, that is required for a T:G to C:G base mismatch correction process. In this study, mutants that affect the level of expression of the two genes were constructed and characterized. Further, a previously isolated mutant, dcm-6, was cloned and mutations within it were identified. Northern blots were used to identify dcm-specific RNA species in wild type and dcm-6 cells. Based on these studies we conclude that there is a six-codon overlap between vsr and dcm. The two proteins appear to be made from a single RNA transcript and translation of dcm is required for the efficient synthesis of Vsr. Further, Vsr is active by itself and may not be produced as a fusion with Dcm. This is the first example of chromosomal genes that overlap in their coding regions and produce proteins with distinct functions.