Abstract
The process of SOS mutagenesis in Escherichia coli requires: i) the replisome enzymes; ii) RecA protein; and iii) the formation of the UmuD'C protein complex which appears to help the replisome to resume DNA synthesis across a lesion. It has recently been shown that the UmuD'C complex, if overproduced, inhibits recombinational repair of a UV-damaged plasmid DNA as well as homologous recombination in an Hfr x F- cross. Since UmuD'C proteins might inhibit an early recombination step by interacting with a RecA nucleo-protein filament, we checked whether UmuD'C proteins will inhibit RecA promoted homologous pairing in vitro. We tested the inhibitory action of UmuD'C proteins in a crude bacterial extract containing possible cofactors such as chaperone proteins that ensure the proper folding of UmuC and the assembly of the UmuD'C complex in vivo. We used a novel recombination assay in which RecA protein promotes the formation of a stable plectonemic joint between a circular single-stranded DNA immobilized onto a membrane and an incoming homologous linear duplex DNA. Under these conditions we show that UmuD'C proteins inhibit the formation of joint molecules.
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