Cross-Specificities Between cII-like Proteins and p_RE-like Promoters of Lambdoid Bacteriophages

Daniel L. Wulff ¹ and Michael E. Mahoney ¹

¹ Department of Biological Sciences, State University of New York, Albany, New York 12222

We have investigated the activation of transcription from the p_RE promoters of phages , 21 and P22 by the and 21 cII proteins and the P22 c1 (cII-like) protein, using an in vivo system in which cII protein from a derepressed prophage activates transcription from a p_RE DNA fragment on a multicopy plasmid. We find that each protein is highly specific for its own cognate p_RE promoter, although measureable cross-reactions are observed. The primary recognition sequence for cII protein on p_RE is a pair of TTGC repeat sequences in the sequence 5'-TTGCN ₆TTGC-3' at the -35 region of the promoter. This same sequence is found in 21 p_RE, while P22 p_RE has the sequence 5'-TTGCN₆TTGT-3', which is the same as that of ctr1, a p_RE⁺ variant of . ctr1 p_RE is half as active as ⁺ p_RE when assayed with either the cII or the P22 c1 proteins. Therefore, the single base change in the P22 repeat sequence cannot explain why the P22 c1 protein is much more active with P22 p_RE than p _RE. The dya5 mutation, a GA change at position -43 of p_RE, makes p_RE a stronger promoter when assayed with either the or 21 cII proteins or the P22 c1 protein. We conclude that efficient activation of a cII-dependent promoter by a cII protein requires sequence information in addition to the TTGC repeat sequences. We do not know the characteristics of the proteins which are responsible for the specificity of each protein for its own cognate promoter. However, dya8, which has a Glu27Lys alteration in the cII protein and a cII⁺ phenotype, results in a mutant cII protein that is much more highly specific than wild-type cII protein for its own cognate p _RE promoter. This is especially remarkable because the dya8 amino acid alteration makes the helix-2 region (the region of the protein predicted to make contact with the phosphodiester backbone of the DNA) of cII protein conform exactly with the helix-2 region of the P22 c1 protein in both charge and charge distribution.

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