Mutations affecting PRM-directed expression of the λ cI-rex operon have been isolated and characterized. Six (class I) mutations, including prm 116 (K.-M. Yen and G. N. Gussin, 1973, Virology 56, 300-312), appear to be defective in the interaction between RNA polymerase and the PRM promoter. Five of the six mutations map in the RNA polymerase "recognition" region (W. Gilbert, 1976, In "RNA Polymerase" (R. Losick and M. Chamberlin, eds.), pp. 193-205. Cold Spring Harbor Laboratory, Cold Spring Harbor, N. Y.) located approximately 35 nucleotides preceding the transcription start point, while one maps near the so-called Pribnow box, a heptanucleotide sequence common to some extent in all promoters sequenced thus far. The existence of mutations in two distinct regions of the promoter may provide the opportunity to distinguish between the functions of the two regions in transcription initiation. Three of the Prm- mutations (class II) are defective in repressor binding at OR. These mutations may be Prm- because repressor is unable to act as a positive regulator of PRM (M. Ptashne, K. Backman, M. Z. Humayun, A. Jeffrey, R. Maurer, B. Meyer, and R. T. Sauer, 1976, Science 199, 156-161). A 10th mutant that exhibited some of the properties expected of a prm- mutant appears to be defective in the cI gene itself; most likely it is a cI missense mutant.
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