Transfer region of IncI1 plasmid R64 and role of shufflon in R64 transfer

Nucleotide sequence and functions of the oriT operon in IncI1 plasmid R64

Two transfer genes of IncI1 plasmid R64, tentatively designated nikA and nikB, were cloned and sequenced. They are located adjacent to the origin of transfer (oriT) and appear to be organized into an operon, which we call the oriT operon. On the basis of the DNA sequence, nikA and nikB were concluded to encode proteins with 110 and 899 amino acid residues, respectively. Complementation analysis indicated that these two genes are indispensable for the transfer of R64 but are not required for the mobilization of ColE1. By the maxicell procedure, the product of nikA was found to be a 15-kDa protein. On treating a cleared lysate prepared from cells harboring a plasmid containing oriT, nikA, and nikB with sodium dodecyl sulfate or proteinase K, superhelical plasmid DNA in the cleared lysate was converted to an open circular form (relaxation). Relaxation of plasmid DNA was found to require the oriT sequence in cis and the nikA and nikB sequences in trans. It would thus follow that the products of nikA and nikB genes form a relaxation complex with plasmid DNA at the oriT site.

Physical and genetic analyses of IncI2 plasmid R721: evidence for the presence of shufflon

A physical map of the 75.1-kb IncI2 plasmid R721 was constructed by using 15 restriction enzymes, and the regions of several genetic determinants including the origins of replication and of conjugal DNA transfer were located on the physical map. It was found that R721 bears a DNA region which undergoes DNA rearrangement similar to the shufflon of R64.