Regulation of UmuD cleavage: Role of the amino-terminal tail

An essential step in SOS mutagenesis is the RecA-mediated posttranslational processing of UmuD-like proteins to the shorter, but mutagenically active, UmuD'-like proteins. Interestingly, the UmuD-like proteins undergo posttranslational processing at different rates. For example, although the Escherichia coli UmuD (UmuD(Ec)) and the Salmonella typhimurium UmuD (UmuD(St)) proteins are 73% identical, UmuD(St) is processed in vivo at a significantly faster rate than the UmuD(Ec) protein. Here, we report experiments aimed at investigating the molecular basis of these phenotypic differences. The faster rate of UmuD(St) cleavage probably does not result solely from a better interaction with RecA, since we observed that, in vitro, UmuD(St) undergoes RecA-independent autocatalytic processing about four-times faster than UmuD(Ec). By constructing chimeric UmuD proteins, we determined that the amino-terminal tail of the UmuD proteins proximal to the Cys24-Gly25 cleavage site is mainly responsible for the difference in UmuD(St) and UmuD(Ec) cleavage rates. Site-directed mutagenesis of the UmuD(Ec) protein suggests that most of the enhanced cleavage observed with the UmuD(St) protein can be attributed to the presence of a Pro23 residue, juxtaposed to the cleavage site in UmuD(St). Furthermore, this proline residue appears to result in a UmuD protein that is a much better substrate for intermolecular cleavage. These findings clearly implicate the N-terminal tail of the UmuD-like proteins as playing an important and unexpected regulatory function in the maturation of the mutagenically active UmuD'-like mutagenesis proteins. (C) 1998 Academic Press.