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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.
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.
Two special genes carried in pairs by the native prophages y and ψ 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 ψ genes anp and skp, are specifically activated by infection of HY cells with κ phage. The κ genes exerting this effect are tay for y and tap for ψ. By means of tay and tap mutants, insertions of phage Mu DNA in the relevant parts of y and ψ 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 ψ. As an accessory result, it provided evidence for a second restriction/modification system. In both cases the O‐specific poly‐saccharides were reduced. The main results of our paper concerning the two pairs of y and ψ genes are as follows: the orders any‐sky and anp‐skp correspond with each other in y and ψ; 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 ψ amount to only about 0.5 kbp. Another observation was that bacteria with a Mu insertion near the skp end of y prophage were no longer cured of ψ when infected with κ tay‐1, in contrast to the efficient curing observed with an ordinary ψ prophage.
The linkage of 34 auxotrophic markers in strain HY of Serratia marce8cens was studied by means of transductions with phages x and y. The markers could be ordered into 18 loci of different auxotrophies by transduction crosses with phage x (markers in trans-position) 17 of which showed to be linked to at least one of the other loci. A linear map of this section of the genome was constructed. It corresponds well to the map of 8 loci derived from transductive crosses (markers in cis-position) with phage y. Since a virion can transduce about 5 loci a t the same time the map represents only a small section of the Serratia genome. No similarity to any part of the maps of Escherichia or Salmonella can be detected. It is discussed why most auxotrophic mutations which were induced by nitrosoguanidine map in this small genomic region. A second transduction system in S. marcescens HY was recently found allowing mapping of markers in trans-position, i.e. using one auxotroph as donor and another one as recipient. This system uses phage kappa ( x ) (ELLMAUER and KAPLAN 1959) which can grow on every substrain of HY because H Y is nonlysogenic for x . The strong killing by x of non-lysogenic recipients (survival due to lysogenization is only STEIGER and KAPLAN 1964) was overcome by using recipients made x-lysogenic. However, x wild type is not suitable as immunizing prophage since it renders the host adsorption resistent. Therefore, the mutant x152 was used which does not inactivate the x-receptors by lysogenic conversion (STEICER 1968). Using auxotrophs of HY (x152) transduction by x can be observed with well measurable frequencies (BAUER 1970, STEIGER et al. 6 0 CUDRUN STAMMINOER, M. BRENUEL and H. w. KAPLAN 1972).Results on co-transductions with this system are reported below. They allowed mapping of a region of the genome containing about 18 auxotrophic loci.Linkage of mutant markers can be demonstrated by co-transduction in two ways: (1) When the two markers (e.g. auxotrophies a and b) are in cis-position in the recipient (ab) and the phage donor was wild type for both of them (a+ b+) co-transduction is indicated by appearance of wild type (a+b+ = fully prototrophic) double transductants a t a measurable frequency somewhat below the frequency of the single transductants (ab+ and a+b). (2) I n the case of transposition of the markers, e.g. if donor is ab+ and recipient a+b, co-transduction is indicated by a reduction of the frequency of wild type (a+b+) transductants compared with the one obtained with wild type (a+b+) donor, since one of the two incorporating crossing overs is restricted to the section between a and b. Further, the frequency of a+b+ transductantsis lower on minimal agar than on minimal agar supplemented with the growth factor necessary for the auxotrophic donor (ab+) since on the supplemented medium an appreciable number of cotransductants (ab+) appear in addition to the single transductants (a+b+). The trans-method has the advantage that every marker in one strain can be tested for linkage with every mar...
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