Growth of [lambda] variants with added or altered promoters in N-limiting bacterial mutants: Evidence that an N recognition site lies in the PR promoter

Abstract

Transcription of the [lambda] genome, initiating at the early rightward promoter (PR), traverses the cII-O-P operon and extends through the Q gene. In the absence of the [lambda] N function, this transcription is prematurely terminated at either of two termination sites, tR1 and tR2. The cII-O-P operon lies distal to tR1, but proximal to tR2. A number of mutations resulting in new promoter activities (e.g., c17 and ric5b) mapping distal to tR1, but proximal to tR2, have been isolated.Although phages carrying the c17 mutation grow in a normal Escherichia coli host, we find that [lambda] derivatives carrying this mutation will not grow in mutant E. coli K12 hosts, Nus-, which limit [lambda] growth by inhibiting the expression of N function. However, under the same conditions, a [lambda] phage containing only the normal [lambda] promoters grows significantly better in the Nus- hosts. Our studies demonstrate that under conditions of limited N expression, phage carrying the c17 mutation can express functions coded for by genes in the cII-O-P operon, but not endolysin, a function coded for by a gene distal to tR2. Thus, under conditions of low N activity, functions whose genes lie downstream from the c17 promoter without any intervening termination signals are expressed. On the other hand, functions whose genes lie downstream from this promoter with an intervening termination signal are not expressed.These results are consistent with a model of N action, which has N acting only with transcription initiating at a specific class of promoters (e.g., PR), c17 not being a member of this class. Although previous studies (Friedman and Ponce-Campos, 1975) have shown that the ric5b promoter is also not a member of the N-utilizing class of promoters, we find that [lambda]ric5b grows on Nus- hosts. This suggests that whereas c17 interferes with transcription from PR, ric5b does not show such an interference.We also find that [lambda] variants carrying two mutations v3 and vs326, which map in the OR - PR region, exhibit the same growth characteristics in Nus- hosts as phages carrying the c17 mutation. These observations imply that the combination of v3 and vs326 interfere with N-modification of transcription initiating at PR, and lead us to conclude that one site for N recognition is located within the PR promoter.