Mating variation by DNA inversions of shufflon in plasmid R64.

Gene organization of the 54-kb transfer region of IncI1 plasmid R64 was deduced from the DNA sequence. Forty-eight ORFs were found in this region. A unique DNA rearrangement designated shufflon is located at the downstream region of an operon responsible for synthesis of thin pilus. The shufflon of R64 consists of four DNA segments, designated as A, B, C, and D, which are flanked and separated by seven 19-bp repeat sequences. Site-specific recombination mediated by the product of the rci gene between any two inverted repeats results in a complex DNA rearrangement. An analysis of open reading frames revealed that the shufflon is a biological switch to select one of seven C-terminal segments of the pilV genes. The products of pilV genes were shown to be components of thin pilus which was required for liquid mating. Seven R64 derivatives where the pilV genes were fixed in the seven C-terminal segments were constructed and their transfer frequencies in liquid mating were measured using various bacterial strains as recipients. Transfer frequencies of R64 in liquid mating strongly depended on the combination of C-terminal segments of the pilV genes in donor cells and bacterial strains of recipient cells, suggesting that the shufflon determines the recipient specificity in liquid mating of plasmid R64.