Two auxotrophic strains of 168 served as recipients for DNA extracted from various wild-type strains of and wild-type species of the genus Depending upon the DNA source, heterologous transformations of the linked loci were either as efficient as those observed with donor DNA obtained from the wild-type 168 strain or were undetectable. The order and relative distances of the three gene loci were the same for all active DNA preparations. Similar results were obtained in heterologous transformations of the linked loci, except that DNA preparations from the and var. species exhibited a reduced but detectable frequency of transformation. With the latter preparations a marked polarity of integration favouring the gene was observed, an effect not seen with homologous DNA. Six independent hybrid lines were obtained from transformation of with DNA from var. . DNA extracted from these lines fell into two classes on the basis of activity in transforming the parental recipient strain: (i) indistinguishable from homologous DNA, and (ii) intermediate between homologous DNA and DNA from the original donor strain. With either class, polarity of integration was no longer observed in the region. The intermediate type of hybrid demonstrates that at least some of the inefficiency of heterospecific transformation must be due to heterology in nucleotide sequence between the different species at the loci.


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