Isolation of a high frequency donor of Rhodobacter capsulatus by integration of the plasmid pMT1000 into the chromosome.

The temperature-independent clone of Rhodobacter capsulatus UA7041, carrying the temperature-sensitive plasmid pMT1000, has been obtained by selection for plasmid markers at the non-permissive temperature. The transfer to Escherichia coli of all drug resistance encoded by pMT1000 was of about 10(-7) when the donor was the UA7041 strain, and of 5 x 10(-4) when the donor was the temperature-sensitive strain of R. capsulatus. Electrophoretic analyses showed no plasmid band typical of the autonomous pMT1000 in the UA7041 strain. Furthermore, UA7041 cells were also able to mobilize chromosomal markers of R. capsulatus at a frequency of about 10(-4) per donor cell. All these results lead us to conclude that the plasmid pMT1000 is integrated in the chromosome of the UA7041 strain. The analysis of the co-transfer frequencies of trp and his markers has shown that the chromosome mobilization in the UA7041 strain of R. capsulatus is unidirectional from a single origin. Our data also show that the pMT1000 plasmid may be useful to the construction of chromosomal, high-frequency donor strains.