Summary: Transposon Tn was shown to be capable of direct conjugative transfer in broth and membrane matings between strains of K12 and between K12 and type b. Only Tn was transferred, but Tn donor ability was not itself inheritable by the recipients and seemed to be associated with the presence of Tn on a non-conjugative pBR322-derived vector in the original donor strain. Transfer of Tn by conjugation was found to be an efficient method for producing insertion mutations in the chromosome of recipient cells. Although such insertions were unstable when the cells were grown under non-selective conditions, it was possible to show that over 40% of the isolated Tn insertions in the chromosome of K12 were in gene(s) concerned with histidine biosynthesis, implying that there is a partial hot-spot for Tn insertion on the K12 chromosome. When a strain of type b was used as a recipient, out of approximately 1500 transconjugants tested, two mutants were isolated with insertions in genes controlling the expression of iron-regulated transferrin-binding proteins. These mutants constitutively produced major 76 kDa and minor 90 kDa proteins which bound transferrin, even when grown under iron-sufficient conditions. Tn insertion mutagenesis, following transfer by conjugation, is a convenient method for isolating mutations in genes concerned with iron acquisition by this important human pathogen.


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