Co‐transduction mapping in Serratia marcescens

Abstract

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...