Two genes in prophage y of Serratia marcescens with functional counterparts in prophage Psi tested by Mu insertions as to order and homology.

Abstract

Two special genes carried in pairs by the native prophages y and psi of Serratia marcescens HY, with functionally rather similar counterparts each, were assigned to restriction fragments and tested for homology. The y genes any and sky, as well as the psi genes anp and skp, are specifically activated by infection of HY cells with kappa phage. The kappa genes exerting this effect are tay for y and tap for psi. By means of tay and tap mutants, insertions of phage Mu DNA in the relevant parts of y and psi prophage, respectively, could be discovered. These insertions and subsequent deletions of Mu were the basis of our studies. The use of Mu was made possible by the isolation of an HY variant giving appropriate plaques with Mu particles of the G(-) type. In the case of another HY variant, Mu plaque formation depended on the presence of a host range mutation isolated by its ability to allow plaque formation on E. coli C, an indicator only for G(-) particles. Unexpectedly, this HY strain was an indicator of both G(-) and G(+) particles, but unfortunately had become adsorption resistant to y and psi. As an accessory result, it provided evidence for a second restriction/modification system. In both cases the O-specific polysaccharides were reduced. The main results of our paper concerning the two pairs of y and psi genes are as follows: the orders any-sky and anp-skp correspond with each other in y and psi; the sky gene, just as the skp gene, lies near one prophage end. However, despite the similarity in function and order, these genes are not homologous, in contrast to any and anp, which are at least partially homologous. The homologous regions of y and psi amount to only about 0.5 kbp. Another observation was that bacteria with a Mu insertion near the skp end of psi prophage were no longer cured of psi when infected with kappa tay-l, in contrast to the efficient curing observed with an ordinary psi prophage.