Essential motifs of relaxase (TraI) and TraG proteins involved in conjugative transfer of plasmid RP4.

Two essential transfer genes of the conjugative plasmid RP4 were altered by site-directed mutagenesis: traG of the primase operon and traI of the relaxase operon. To evaluate effects on the transfer phenotype of the point mutations, we have reconstituted the RP4 transfer system by fusion of the transfer regions Tra1 and Tra2 to the small multicopy replicon ColD. Deletions in traG or traI served to determine the Tra phenotype of mutant plasmids by trans complementation. Two motifs of TraG which are highly conserved among TraG-like proteins in several other conjugative DNA transfer systems were found to be essential for TraG function. One of the motifs resembles that of a nucleotide binding fold of type B. The relaxase (TraI) catalyzes the specific cleaving-joining reaction at the transfer origin needed to initiate and terminate conjugative DNA transfer (W. Pansegrau, W. Schröder, and E. Lanka, Proc. Natl. Acad. Sci. USA 90:2925-2929, 1993). Phenotypes of mutations in three motifs that belong to the active center of the relaxase confirmed previously obtained biochemical evidence for the contributions of the motifs to the catalytic activity of TraI. Expression of the relaxase operon is greatly increased in the absence of an intact TraI protein. This finding suggests that the relaxosome which assembles only in the presence of the TraI in addition to its enzymatic activity plays a role in gene regulation.