Probing cII and himA action at the integrase promoter pi of bacteriophage lambda.

Plasmids were constructed to supply cII-coded protein for activation of the phage promoter pI. Using a fusion which expresses lacZ from pI. We can accurately follow activation of pI without having to assay int activity in vivo. A large excess of cII protein compared to a normal lytic infection stimulates lacZ expression about 10-fold over the basal level. The int-c226 constitutive allele of pI is not further activated by cII even though its level of lacZ expression is less than the maximal cII-activated expression from wild type pI. A himA-deleted strain also shows activation, demonstrating that there is no absolute requirement for the himA gene product for cII-stimulated transcription at pI.