Cooperative effects of bacterial mutations affecting [lambda] N gene expression : II. Isolation and characterization of mutations in the rif region

Abstract

We report the isolation and initial characterization of a class of mutations, Snu, that map near the rif locus on the Escherichia coli chromosome. Snu mutations inhibit the growth of phage [lambda], an effect primarily seen when Snu mutations are combined with another class of mutations, nus. Nus mutations have previously been shown to inhibit the expression of the N gene product of [lambda], and the experiments reported here suggest that Snu mutations add to this inhibitory effect.One Snu mutation, Snu-9, was shown to cause bacterial growth to be temperature-sensitive. This suggests that, at least, some Snu mutations may be in genes coding for a function essential for bacterial growth. Since genes coding for the [beta] and [beta]' subunits of RNA polymerase map in this region, we recognize that Snu mutations might alter either of these subunits of RNA polymerase. Complementation studies demonstrate that Snu+ is dominant to Snu-, indicating that the mutant phenotype is due to the partial loss of a function necessary for full N expression.Although the hosts carrying Snu and nus mutations (called Supernus) severely restrict the growth of phage which express the N function of [lambda], they do not show any increased inhibitory effect on the growth of [lambda]immP22 and [lambda]imm21, phages which express N functions different from that of [lambda]. However, Supernus hosts do restrict the growth of a [lambda] variant that can grow well in bacteria carrying either component mutation, Snu- or nus-. The restrictive effect of the Supernus strain is far greater than would be expected if the restriction was due to an additive effect of the two component mutations. This implies that there might be an interaction between Snu and nus products and that the Supernus phenotype results from an interference with this interaction.