Physical characterization of non-conjugative citrate utilization plasmids.

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Abstract
Seven citrate-utilizing (Cit+) Escherichia coli strains originated from horses (2 strains) and cattle (5 strains) were examined for transferability of the Cit+ character and the presence of bacterial plasmids. All 7 strains harbored plasmids in the cell, although the Cit+ determinants in the Cit+ E. coli strains tested could not be transferred to E. coli K-12 strain. Two horse strains possessed .apprx. 60 megadaltons plasmids; the plasmid in strain OH3026 was thought to be a non-conjugative Cit plasmid, but that in OH3025 was not shown to encode the Cit+ functional ability. Plasmid DNA prepared from 2 strains (OH3035 and OH3036) of 5 bovine strains could produce Cit+ transformants, but no Cit+ transformant was obtained in the remaining 3 strains. The representative non-conjugative Cit plasmid, pOH3035 (20.5 kb [kilobase]), prepared from the Cit+ transformant of pOH3035 was examined by restriction endonuclease, and its EcoRI-cleaved B fragment (5.5 kb) carrying the Cit+ functional gene was cloned to a vector plasmid pBR322. The Cit+ recombinant plasmid, pOH8, constructed by cloning was analyzed for potential transposability of Cit+ gene, but the transposable element coding for the Cit+ determinant was not obtained from pOH3035 and pOH8 in this study. The restriction map of pOH3035 was also compared with that of a plasmid pBR322 carrying the citrate utilization transposon, pOH2, and the relative similar order of recognition sites for BamHI and Bg/II enzymes was found on the Cit+ functional gene of the 2 plasmids.