Genetic Exchange in Escherichia Coli K 12

Abstract

The finding that the genetic contribution of the F+ parent is on the avg. much less than that of the F- parent (F+ polarity) is discussed in relation to genetic segregation. In order to distinguish recombinant cells, the parental cells are labeled with selective nutritional markers, which by recombination give rise to prototrophs, and also with unselected non-nutritional markers. Assuming that each haploid nucleus has several non-homologous chromosomes and that zygotes are usually formed by the union of one F+ chromosome with the full complement of F- chromosomes, the recombinants will derive most of their genes from the F- parent. Two types of linkage will be observed, between genes on the same chromosome, and between genes on non-homologous chromosomes as a result of incomplete participation of the F+ genome. In the latter case linkage will be between genes on the unrecombined chromosome(s) and the selective marker(s) of the F- parent. Situations are discussed in which the selective markers of the parental strains are located on homologous chromosomes and on non-homologous chromosomes. The expts. dealt with crosses between 2 complementary bacterial strains derived from K-12, using as selective markers nutrient requirements for methionine, leucine, threonine, B1 and lactose as well as resistance to azide and streptomycin. Considering the appearance of an unselected F+ allele in a prototroph as an indication of linkage to an F+ selective marker, 3 linkage groups were indicated. In the cross 58-161 F-xW677 F+ in which methionine was the F+ selective marker, B1 was the only unselected'' marker linked to methionine. In the reciprocal cross 58-161 F+ x W677 F-, using threonine and leucine as selective markers, 3 unselective markers (resistance to azide and to phage T1, and requirement for lactose) were linked to threonine or leucine. The remaining unselective markers (resistance to streptomycin and to phage T3, and requirement for maltose and for mannitol) were not selected for by leucine, threonine or methionine, indicating the presence of one or more additional linkage groups. From these results and those of other workers it was concluded that the decreased genetic contr. of the F+ parent was not due to repeated elimination of a specific fraction of the F+ genome, and that the F+ loci were transferred as groups of linked genes. Preliminary tests support the theory that the F+ contr. occurs in the form of discrete chromosomes and not as chromosomal fragments randomly formed during recombination. The reason for the infrequent appearance within prototrophs of unselected F+ loci is not clear, although it is known that K-12 strains differ among themselves in the frequency with which they transfer unselected loci and it is thought that the prototrophs are not due entirely to polarity reversal during a cross.