Isolation and characterization of Hfr strains of Erwinia amylovora

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Abstract
Hfr strains (Hfr 159 and its derivatives, Hfr 160 and Hfr 161) were constructed from Erwinia amylovora ICPB EA178 by introducing an Escherichia coli F′his+ plasmid and then selecting for integration of F′his+ after treatment with acridine orange. The Hfr strains were relatively stable upon repeated transfers on nonselective media. Interrupted mating experiments and analyses of inheritance of unselected markers showed that his+ is transferred by Hfr 159 as the proximal marker at a relatively high frequency (about 5 × 10−4 recombinants per input donor cell), followed by ilv+, orn+, arg+, pro+, rbs+, met+, trp+, leu+, ser+, and thr+ (not necessarily in that precise order). The donor strains, previously constructed in E. amylovora by integration of F'lac+ from E. coli, transfer cys+ as the proximal marker followed by ser+. Further analysis of one of those earlier donor strains, Hfr 99, showed that ser+ is followed by arg+, orn+, met+, pro+, leu+, ilv+, rbs+, his+, trp+, and thr+ (not necessarily in that precise order). Thus, the Hfr strains constructed by integration of F′his+ are different, in terms of origin and direction of transfer, from those derived from integration of F′lac+. The applicability of these Hfr strains to mapping the genes on the E. amylovora chromosome is indicated.

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