Selective inhibition of Escherichia coli recBC activities by plasmid-encoded GamS function of phage lambda

The gam locus of bacteriophage lambda encompasses two coding sequences with the same reading frame and translational stop, one corresponding to an Mr 11646 polypeptide (gamS gene), the other to an Mr 16349 polypeptide (gamL gene). A DNA segment encoding gamS but not gamL was placed under lambda pR promoter control (regulated by the cIts857-coded repressor) on a multicopy plasmid, and an insertion mutation (gamS201) was constructed. Expression of gamS+, but not gamS201, inhibited Escherichia coli RecBC nuclease in vivo; the criteria were inhibition of chromosomal DNA degradation after UV irradiation and plating of T4 gene 2- phages. The recB+ C+ bacteria expressing gamS+ were completely or partially similar to recC- mutants with respect to certain phenotypes: defective plating of phages P1 and P2, ability to plate (in a recA- background) lambda red- gam- phages, reduced resistance to UV irradiation, defective SOS induction, decreased colony-forming ability.