One of the components of the RecA-LexA-controlled SOS response in Escherichia coli cells is an inducible error-prone DNA replication pathway that results in a substantial increase in the mutation rate. It is believed that error-prone DNA synthesis is performed by a multiprotein complex that is formed by UmuC, UmuD, RecA, and probably DNA polymerase III holoenzyme. It is postulated that the formation of such a complex requires specific interactions between these proteins. We have analyzed the specific protein-protein interactions between UmuC, UmuD, and UmuD fusion proteins, using a Saccharomyces cerevisiae two-hybrid system. In agreement with previous in vitro data, we have shown that UmuD and UmuD are able to form both homodimers (UmuD-UmuD and UmuD-UmuD) and a heterodimer (UmuD-UmuD). Our data show that UmuC fusion protein is capable of interacting exclusively with UmuD and not with UmuD. Thus, posttranslational processing of UmuD into UmuD is a critical step in SOS mutagenesis, enabling only the latter protein to interact with UmuC. Our data seem to indicate that the integrity of the entire UmuC sequence is essential for UmuC-UmuD heterotypic interaction. Finally, in our studies, we used three different UmuC mutant proteins: UmuC25, UmuC36, and UmuC104. We have found that UmuC25 and UmuC36 are not capable of associating with UmuD. In contrast, UmuC104 protein interacts with UmuD protein with an efficiency identical to that of the wild-type protein. We postulate that UmuC104 protein might be defective in interaction with another, unknown protein essential for the SOS mutagenesis pathway.Exposure of Escherichia coli cells to agents that damage DNA and introduce replication-blocking lesions results in the induction of the SOS regulon (13,22,33,51,52). Approximately 20 cellular genes are derepressed when activated RecA promotes the proteolytic cleavage of LexA protein, the repressor of SOS genes (22,25,32,33,44,51,52).One of the components of the RecA-LexA-controlled SOS response is an inducible error-prone DNA replication pathway that causes a significant increase in the mutation rate (13,22,33,51,52). Genetic and physiological experiments indicate that the products of the umuDC operon and the recA gene are essential components of the SOS mutation pathway (3,12,16,17,26,44,47,48). This is supported by the fact that umuD and umuC mutants of E. coli are virtually nonmutable with UV light and many chemical agents (26,47).The recA and umuDC genes are members of the SOS regulon and are expressed at elevated levels after DNA damage (15,31,46,53,55). In SOS-competent cells, activated RecA also mediates the cleavage of UmuD at its Cys-24-Gly-25 bond, yielding a shorter UmuDЈ product, which has been proposed to be an active form of UmuD for mutagenesis (6,37,45,54). Several data suggest that UmuC, UmuDЈ, and RecA proteins facilitate the proceeding of the DNA replication complex beyond lesions in the template (5, 21, 40, 41). In addition, it has been shown that overexpression of the umuDC operon results in cold sensitivit...