Evidence that the cro repressor inhibits expression of the bacteriophage λP gene at high multiplicities of infection

Summary

The activity of the λP gene product at various multiplicities of infection (m.o.i.) was examined in C ^-₁ 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 λC₁857 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 λC_I857C_II2002 or λC_I857cro27 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 λC_I857cro27susP3. At high m.o.i., the cleavage of c.c. DNA was inhibited after infection with λC_I857C_II2002, but not after infection with λC_I857cro27. It is concluded that at high m.o.i. in C ^-₁ infections, the expression of gene P is unaffected by the C_II gene product, but is inhibited by the increased intracellular levels of cro protein.