Nonsense mutants of Serratia phage Kappa. I. Their isolation and use for proving circular linkage of the phage genes

Involvement of two genes of superinfecting phage kappa in curing and induction of prophage psi in Serratia marcescens HY

Prophage psi carried along with prophage y by Serratia marcescens HY is subject of moderate curing at heteroimmune superinfection of cells from stationary phase with phage kappa. Curing becomes considerably more frequent when the bacteria are non-lysogenic for y. Both psi, y-double-lysogenic and psi-single-lysogenic cells with a mutation in the ink gene are very efficiently cured of psi if infected by kappa tay, although this mutant was characterized as being deficient in transactivation of certain genes in prophage y. On the other hand to get efficiently cured after kappa wild-type infection these cells too must be devoid of a y prophage. Thus a y function turned on by tay+ seems to counteract the elimination of psi. However, interestingly enough psi curing is boosted by a further y function under special circumstances. Efficient curing depends on an intact kappa tap gene, a gene reported to cause transactivation of certain psi genes. Curing at kappa tay infection is specifically accompanied by induction of the psi prophage in a part of the infected cells. However, there is no such induction at kappa wild-type infection, either in the absence or presence of a y prophage. An explanation of these findings is suggested which includes an antirepressive effect exerted on psi and a hypothetical interaction between the products of genes tap and tay.

Genetic studies of the ends of a locked-in kappa prophage in Serratia marcescens by transductional and vegetative crosses

In temperate phage kappa of Serratia marcescens several special features of different phages are combined. The unessential genes lI, iny, cII and, at least to some extent, even the integrase gene int are not subject to negative control by the repressor, the product of gene cIII. 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 cIII 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 kappa's genetic map. The right prophage end appears to face at least two trp cistrons, among them the gene encoding anthranilate synthetase. lI 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 cII is still unknown, but some of its mutants display a cold-sensitive phenotype, their plaques being clear at 30 degrees C and turbid at 37 degrees C. Bacteria with such prophages stop producing viable progeny when the cultures are shifted from 37 degrees C to 30 degrees 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.