SPECIFIC IN-VIVO PROTEIN-PROTEIN INTERACTIONS BETWEEN ESCHERICHIA-COLI SOS MUTAGENESIS PROTEINS

One of the components of the RecA-LexA-controlled SOS response in Esch erichia coli cells is an inducible error-prone DNA replication pathway that results in a substantial increase in the mutation rate. It is be lieved that error-prone DNA synthesis is performed by a multiprotein c omplex that is formed by UmuC, UmuD', RecA, and probably DNA polymeras e III holoenzyme. It is postulated that the formation of such a comple x requires specific interactions between these proteins. We have analy zed the specific protein-protein interactions between UmuC, UmuD, and UmuD' fusion proteins, using a Saccharomyces cerevisiae two-hybrid sys tem. 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 prote in is capable of interacting exclusively with UmuD' and not,vith UmuD. Thus, posttranslational processing of UmuD into UmuD' is a critical s tep in SOS mutagenesis, enabling only the latter protein to interact w ith UmuC. Our data seem to indicate that the integrity of the entire U muC sequence is essential for UmuC-UmuD' heterotypic interaction. Fina lly, in our studies, we used three different UmuC mutant proteins: Umu C25, UmuC36, and UmuC101. We have found that UmuC25 and UmuC36 are not capable of associating with UmuD'. In contrast, UmuC104 protein inter acts with UmuD' protein with an efficiency identical to that of the wi ld-type protein. We postulate that UmuC104 protein might be defective in interaction with another, unknown protein essential for the SOS mut agenesis pathway.