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https://www.ias.ac.in/article/fulltext/jgen/077/02-03/0105-0114

adaptive mutagenesis; genome evolution; Escherichia coli ; CHI sequence; mismatch repair; cytosine; methylation

Recently we proposed that sequences in the immediate neighbourhood of cytosine residues whose sequence context permits their methylation by DNA cytosine methyltransferase (Dcm) experience hypermutagenesis in cells exposed to nonlethal stresses. This hypothesis could explain the peculiar spectrum of the late-arising Lac⁺ mutants seen in theE. coli strain FC40. Here we present results of computer analysis which show that Dcm substrate sequences are overrepresented in theE. coli genome. Interestingly, certain noncanonical Dcm sequences are more overrepresented than the canonical one. The most overabundant of these, DCM-III (5’ GCTGG3’), forms the 5’ end of the recombinogenic octamer CHI (5’ GCTGGTGG3’). CHI is even more overrepresented than DCM-III. We propose that the overabundance of the DCM and CHI sequences is due to their ability to enhance adaptive fitness of the host by inducing hypermutagenesis in cells exposed to nonlethal, growth-blocking stresses. The CHI context seems to stimulate the adaptive activity of DCM-III by a mechanism which may not directly involve its recombinogenic activity.

S. K. Mahajan¹ N. D. Shirke² S. K. Bhattacharjee¹

1. Molecular Biology and Agriculture Division, Mumbai - 400085, India
2. Computer Division, Bhabha Atomic Research Centre, Mumbai - 400085, India

Published

12 August 1998

Manuscript received

3 August 1998

Manuscript revised

7 November 1998