GENETIC AND MOLECULAR ANALYSES OF THE C-TERMINAL REGION OF THE RECE GENE FROM THE RAC PROPHAGE OF ESCHERICHIA-COLI K-12 REVEAL THE RECT GENE

The nucleotide sequence of the C-terminal region of the recE gene of t he Rac prophage of Escherichia coli K-12 reveals the presence of a par tially overlapping reading frame we call recT. Deletion mutations show that recT is required for the RecE pathway of conjugational recombina tion. By cloning recT with a plasmid vector compatible with pBR322, we showed by cis-trans tests that the portion of the recE gene encoding ExoVIII DNA nuclease activity is also required for RecE pathway conjug ational recombination. The recT gene can replace the redB gene of lamb da for recA-independent plasmid recombination. A Tn10 insertion mutati on previously thought to be in recE is located in recT and is renamed recT101::Tn10. Discrepancies between the molecular mass estimates of w ild-type ExoVIII protein determined from mobility in sodium dodecyl su lfate-polyacryl-amide gel electrophoresis (SDS-PAGE) and calculated fr om the predicted amino acid sequence are discussed. The hypothesis tha t wild-type ExoVIII protein results from fusion of RecE and RecT prote ins is disproved genetically, thus supporting a previous hypothesis th at the discrepancies are due to abnormal protein mobility in SDS-PAGE. A computer-performed scan of the bacteriophage nucleotide sequence da ta base of GenBank revealed substantial similarity between most of rec E and a 2.5-kb portion of the b2 region of lambda. This suggests inter esting speculations concerning the evolutionary relationship of lambda and Rac prophages.