Micrococcus luteus homolog of the Escherichia coli uvrA gene: identification of a mutation in the UV-sensitive mutant DB7

Abstract

Restriction fragments of Micrococcus luteus DNA containing the gene affected by a mutation in the UV-sensitive mutant DB7 were cloned both from the wild type and from the mutant in an Escherichia coli host-vector system. The wild-type fragment was able to reverse the multiple sensitivity of the mutant to UV, mitomycin C, and 4-nitroquinoline 1-oxide by a one-step transformation. Determination of the nucleotide sequences revealed a potential open reading frame coding for a protein of 992 (tentative) amino acid residues, within which the DB7 mutation was identified as a CG-to-TA transition causing a translation termination. The putative product of the open reading frame shares an extensive amino acid sequence homology with the E. coli UvrA protein comprising 940 residues. The homology extends over the greater part of both polypeptides except for two extra sequences of 31 and 24 amino acid residues located at the amino-terminal and in the interior, respectively, of the M. luteus protein. In the homologous region, 56.7% and 16.7% of the 933 pairs of the aligned amino acids were accounted for by conserved residues and conservative substitutions, respectively. These results indicate that the gene defined by the mutation in DB7 represents a homolog of the E. coli uvrA gene. Hence, it has to be concluded that DB7, known for its deficiency in UV endonuclease (pyrimidine dimer DNA glycosylase/apurinicapyrimidinic endonuclease) activity, is a double mutant which is also defective in an enzyme complex similar to the E. coli UvrABC excinuclease.