A $80 Function Inhibitory for Growth of Lambdoid Phage in Him Mutants of Escherichia co/i Deficient in Integration Host Factor

Bacteriophage $80 and h-480 hybrid phage of the type X(QSR),, in which the rightmost 10% of the X genome is replaced by corresponding 480 material, are unable to grow lytically in himA and hip/himD mutants of Eschwichia cdi K12 at 32”. The genetic element responsible for the growth defect, rha, has been mapped to the (QSR)m region and was located more precisely by restriction enzyme and DNA heteroduplex analysis of mutations that result in loss of the Rha phenotype. Such an Rha mutant carrying a 1.5-kb deletion beginning 0.58 kb from the right end of the chromosome and extending leftward locates the rhu locus at least in part within this region of (QSR)a. In addition, a substitution derivative of h(QSR), was isolated which does not exhibit the Rha phenotype. In this phage, X-8Ohy95, the right half of the (QSR)a region is replaced by DNA homologous to the 95100% segment of X. In mixed infections in the himA host at 32”, X+ does not complement X(QSR)m for growth and the burst size of the coinfecting X+ is reduced in comparison to that in a single infection. Deletion mutants of X(QSR)80 that grow normally in himA at 32’ in single infections are inhibited for growth in mixed infections with X(QSR)W. These results suggest the existence of a tram-acting function which inhibits phage growth in the absence of HimA or Hip/HimD function. It is likely that the rha gene either encodes that function or indirectly controls its action.