Interaction of cII, cIII, and cro gene products in the regulation of early and late functions of phage λ

Strains of λ phage containing a mutated cII, cIII, or cy gene in addition to a cro^- mutation were constructed. Each single mutant and the double mutants were analyzed for growth behavior and for expression of specific early and late λ proteins. λc1857cro 16 is unable to grow well at 30° and at 43°. This strain is incapable of turning off early genes (of the b region, of the P_L and P_R operons, and the cU and int genes) and is defective in the expression of late functions. The clear mutations permit phage growth at 30°. They relieve the inhibitory effect conferred by the cro^- mutation on phage growth at 43° to different extents. Our results also indicate that the cII and cIII proteins do not necessarily act at y in the regulation of late functions.