TRANSFER PROTEIN TRAM STIMULATES TRAI-CATALYZED CLEAVAGE OF THE TRANSFER ORIGIN OF PLASMID R1 IN-VIVO

Factors contributing directly to the cleavage of the conjugative trans fer origin of plasmid R1 in Escherichia coli were investigated. The es sential transfer protein TraM was identified as a necessary positive e ffector of the catalytic activity of TraI relaxase at the R1 transfer origin in the absence of protein TraY. The stimulatory effect of TraM on the cleavage reaction in vivo correlated with the capacity of TraM to bind origin DNA. TraM was shown to be essential for heterologous mo bilization of recombinant origin DNA. The requirement for TraM to prom ote mobilization was distinct from the protein's positive effect on tr ansfer gene regulation. Chimeric traM alleles, fusing heterologous ami no and carboxyl coding sequences from the traM genes of the R1 and the IncFI plasmid P307, were used to localize the specificity determinant of TraM's DNA binding activity. Use of the chimeric alleles also reve aled that the requirement for TraM in mobilization is origin specific but transfer system independent. No evidence was found for a plasmid s pecific activity of TraM at a stage in the transfer process subsequent to the initial cleavage of origin DNA. In light of TraM's regulatory functions in transfer gene expression, we propose that TraM could cont rol conjugative DNA processing in response to intracellular levels of transfer proteins. (C) 1998 Academic Press Limited.