The repair ability for UV-induced damage observed for Streptococcus pneumoniae proceeds through a system similar to the Uvr-dependent system in Escherichia coli. The DNA sequence of a gene complementing uvr-402, a mutation conferring UV sensitivity, was determined. Alignments of the deduced amino acid sequence revealed an extensive sequence homology of 55% with the UvrB protein of E. coli and 59%o with the UvrB-homologous protein of Micrococcus luteus. Nucleotide-binding site consensus was observed. The high conservation of the uvrB-like gene among these three species suggests that the role of the UvrB protein and excision repair in general might be very important for cell survival.

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The mbrC17 mutation in Escherichia coli had been shown to cause conditional growth defects and an increase in the quantity of DNA per cell. The present work was aimed at identifying the mutation. Sequencing showed that the MbrC17 phenotype does not involve glr (murI), as previously suggested. P1 transduction data indicated that the mbrC17 mutation is closely linked to rpoB, and allele exchange showed it to lie within the secE-nusG operon. A single change relative to wild type was found in the secE-nusG region from the mbrC17 strain, a G-->A mutation 23 bp upstream of the secE coding sequence. This mutation causes a two-fold increase in the concentration of secE-nusG mRNA.

An Escherichia coli mutant lacking deoxycytidine triphosphate deaminase (Dcd) activity and an unknown function encoded by a gene designated ior exhibits sensitivity to ionizing radiation whereas dcd mutants themselves are not sensitive. A DNA fragment from an E. coli genomic library that restores the wild type level of UV and gamma ray resistance to this mutant has been cloned in the multicopy vector pBR322. Comparison of its restriction map with the physical map of the E. coli chromosome revealed complete identity to the recBD genes. ior affects ATP-dependent exonuclease activity, suggesting that it is an allele of recB. This mutation alone does not confer sensitivity to UV and gamma radiation, indicating that lack of Dcd activity is also required for expression of radiation sensitivity.