Evidence that the cro repressor inhibits expression of the bacteriophage lambda P gene at high multiplicities of infection.

The activity of the lambda P gene product at various multiplicities of infection (m.o.i.) was examined in CI- conditions using an assay which measures the disappearance of the rapidly-sedimenting closed-circular (c.c.) form of phage DNA. When cells were infected with lambda CI857 at multiplicities of 5 phage/cell or less, between 65%-75% c.c. DNA was lost during incubation. If the multiplicity of infection was increased to 10 phage/cell or greater, a marked inhibition in the cleavage of c.c. DNA was observed. When bacteria were infected with either lambda CI857CII2002 or lambda CI857cro27 at low m.o.i., the usual 65%-75% decrease in the percentage of c.c. phage DNA occurred during incubation. In contrast, no losses in c.c. DNA were noted after infection with lambda CI857cro27susP3. At high m.o.i., the cleavage of c.c. DNA was inhibited after infection with lambda CI857CII2002, but not after infection with lambda CI857cro27. It is concluded that at high m.o.i. in CI- infections, the expression of gene P is unaffected by the CII gene product, but is inhibited by the increased intracellular levels of cro protein.