Lac DNA, RNA polymerase and cyclic AMP receptor protein, cyclic AMP, lac repressor and inducer are the essential elements for controlled lac transcription

Adenosine 3':5'-cyclic monophosphate concentration in the bacterial host regulates the viral decision between lysogeny and lysis

Mutants of Salmonella typhimurium defective in adenylate cyclase (cya gene) or in cAMP receptor protein (crp gene) are lysogenized at reduced frequency by phage P22. One class of the bacterial mutants with an altered RNA polymerase (rif gene) is also lysogenized at reduced frequency. In the three types of mutant bacteria, the phage's decision between lysogeny and lysis is shifted to lysis and the phage form clear plaques. We propose that in wild-type bacteria the cAMP-receptor protein, in combination with cAMP, activates bacterial RNA polymerase to transcribe certain phage genes that are required for efficient lysogenization. Under conditions of strong catabolite repression, when the supply of energy and biosynthetic components is abundant and the concentration of cAMP is low, the phage would multiply and lyse the cell. When the supply of energy is deficient and the concentration of cAMP is high, the phage would lysogenize the cell. Phage mutants have been isolated that form turbid plaques on the three classes of bacterial mutants due to a higher frequency of lysogeny. These phage mutants have been shown by complementation to be defective in the same gene, which we have called the cly gene. These cly mutants lysogenize the wild-type bacteria with a 99% frequency and, thus, do not form plaques on them. Other kinds of bacterial mutants are also lysogenized at reduced frequency by phage P22. They may be altered in other physiological control systems that influence the frequency of lysogenization.

Mechanism of initiation and repression of in vitro transcription of the lac operon of Escherichia coli

A cyclic AMP-binding protein (CAP protein), cyclic AMP, and RNA polymerase holoenzyme are shown to initiate lac transcription at the lac promoter. Lac repressor appears to control transcription by preventing RNA polymerase and/or CAP protein from binding to the lac promoter. Results support the idea that the lac promoter is composed of two sites that interact with CAP protein and RNA polymerase holoenzyme. The promoter can be altered by mutation so that holoenzyme alone can initiate lac transcription correctly.