We report the isolation and characterization of an Escherichia coli mutant which limits the growth of phage A by inhibiting the expression of the N gene regulatory function. The mutation involved maps near minute 11 of the E. coli chromosome smd dominance tests show that the mutant allele is recessive to the wild one. Therefore, we conclude that the locus involved normally codes for a function necessary for N expression. Another mutant which exhibits a similar phenotype has previously been reported and the mutation involved, in that case, maps at minute 61. This mutant is called Nus (N utilization substance); we have named the locus at minute 61 nusA, and the locus at minute 11, nusB. Although the nusA allele is not found in Salmonella typhosa, our studies demonstrate that the nusB allele is found in this closely related enterobacteriaciae. A nusA-1 nusB-5 double mutant was constructed and exhibited a far more restrictive effect on h growth than either of the single nus mutants. Further, we have constructed a A variant which carries the nusB+ allele. This phage plates on nusB-5 mutants under restrictive conditions, but not on the nusA-1 mutants.

Mutations in two Escherichia coli genes, sipA and sipB, result in a specific inhibition of the growth of certain hybrid lambdoid bacteriophages, A immRI2, that have the early regulatory regions and adjacent genes from bacteriophage P22. The sipB391 mutation maps near minute 56 and exerts the strongest inhibitory effect on the growth of the hybrid phages. The sipAl mutation maps near ninute 72 and plays an auxiliary role: enhancing the action of sipB391. Such a role is not limited to sipAl, since there is a similar enhancement by the nusAl and nusE71 mutations. The Sip-imposed restriction on the growth of X immP22 phages is not observed if the phage carries a mutation in the cl gene. Perhaps this reflects the fact that the cl product regulates phage DNA replication and is a major determinant in the decision governing whether the phage takes the lytic or lysogenic pathway. Consistent with this idea is the observation that A immP2 DNA replication is severely inhibited in bacteria carrying the sipB391 mutation. It is suggested that sip mutations exaggerate the normal role of cl in limiting lytic growth. This causes a failure in the expression of sufficient amounts of some or all of the lytic gene products required for phage growth.Host mutants that influence the growth of lambdoid bacteriophages have served as remarkable tools both for studying the action of phage gene products and for identifying genes whose products play important roles in the physiology of the host (19). One class of host factors is those that influence phage products active in the process of establishing repressor synthesis. The hfl (high frequency of lysogeny) (4, 5; F. Banuet and I. Herskowitz, personal communication) and the him and hip (host integration mediation and host integration protein) (33,41,44) genes control the level of cII protein, a bacteriophage X protein that plays a pivotal role in the decision determining whether the infecting phage follows the lytic or lysogenic route. This regulation is exerted in three ways: (i) controlling the establishment of cI (repressor) expression (26, 56); (ii) stimulating expression of the gene int, whose product is required for prophage integration (16); and (iii) reducing expression of late genes (13,40).The cl gene of lambdoid bacteriophage P22 maps in the same relative position as the cIT gene of X and encodes a protein that serves functions analogous to those of the cII protein (35, 52). P22, whose natural host is Salmonella typhimurium, is unable to infect Escherichia coli because it is unable to adsorb. Hybrid phages that are able to grow in E. coli have been engineered under special conditions by using in vivo recombination (10, 23, 28, 58). Some of these hybrids carry the early regulatory and replication genes of P22 and the late morphogenic genes of X including those encoding tail proteins. The relevant genetic structures of X and P22 are shown in Fig. 1. The hybrids used in our experiments carry the early regulatory region from P22 (immC), which includes the c2 (repressor) gene, and t...

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 A, 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 h, 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 temperaturesensitive. 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 p and p' 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 A, they do not show any increased inhibitory effect on the growth of himmP22 and Aimm21, phages which express N functions different from that of A. However, Supernus hosts do restrict the growth of a A 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. I Author to whom reprint requests should be ad-been identified, that being the p subunit of dressed.

We also find that A variants carrying two mutations v, and vsgz6, which map in the 0,-Ps region, exhibit the same growth characteristics in Nus-hosts as phages carrying the cl7 mutation. These observations imply that the combination of vg and vsR16 interfere with N-modification of transcription initiating at Pa, and lead us to conclude that one site for N recognition is located within the Pa promoter.

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