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.