SPECIFIC REPRESSION OF INVITRO TRANSCRIPTION BY THE CRO REPRESSOR OF BACTERIOPHAGE-LAMBDA

The Cro protein specified by bacteriophage λ is a repressor needed for regulation of lytic growth. The experiments presented here show that Cro represses specifically transcription in vitro of the N, cI and fro genes, initiated at the promoter sites pL, prm and pR, respectively, but does not affect transcription of 4 S RNA (also called oop RNA) and 6 S RNA. To carry out its repression function Cro protein must interact with the operator sites, ol and or, since mutations in oL and oR abolish or reduce Cro-mediated repression. The interaction of Cro with the OR region (oR1oR2oR3) was studied in detail, using a restriction fragment which carries parts of the cI and the cro genes with the promoter-operator region in the middle. Cro protein was shown to have functional diversity: at low concentrations, Cro represses transcription of the cI gene and slightly stimulates transcription of the cro gene; at higher concentrations, Cro represses transcription of the cro gene. From these experiments, we argue that Cro binds to the oR3 site first and then occupies the oR2 and o sites, a binding sequence opposite to that of cI repressor. The two repressors, Cro and cI, compete for the operator sites at the appropriate concentration. These results can explain the immune and anti-immune states demonstrated in vivo for phage λ and also provide a general model for quasi-stable regulatory switches. © 1979.