Genetic studies of the ends of a lacked‐in Kappa prophage in Serratia marcescens by transductional and vegetative crosses

In temperate phage x of Serratia marcescens several special features of different phages are combined. The unessential genes l I , iny , c II and, at least to some extent, even the integrase gene int are not subject to negative control by the repressor, the product of gene c III . A genetic map of the prophage was established using defective, heat‐induced lysates of int − lysogens both in vegetative crosses with sus mutants of essential genes and in transduction of the four unessential genes to lysogenic recipients. Results from reciprocal four factor‐crosses concerning the order of the four genes had to be included. The four genes are located near the right end of the prophage, whereas c III lies near its left end. In vegetative phage all five genes lie in an interval between the essential genes T and U , comprising 10% of x 's genetic map. The right prophage end appears to face at least two trp cistrons, among them the gene encoding anthralinate synthetase. l I encodes a product that masks the phage receptors in the cell wall. The gene product of iny interferes with the growth of infecting phage y. The natural function of c II is still unknown, but some of its mutants display a cold‐sensitive phenotype, their plaques being clear at 30°C and turbid at 37°C. Bacteria with such prophages stop producing viable progeny when the cultures are shifted from 37°C to 30°C. These cold‐sensitive mutants are partly dominant and partly recessive. Analysing a virulent mutant, a gene ant encoding an antirepressor was discovered, but so far there is no evidence that it is regulated by an extra repressor. The gene is located relatively near the left prophage end. Evidence is presented that the exogenotes in transduction with the defective lysates continue to exist for some time after a first recombinational event.