RecA protein and SOS. Correlation of mutagenesis phenotype with binding of mutant RecA proteins to duplex DNA and LexA cleavage

Overproduction of SSB protein enhances the capacity for photorepair in Escherichia coli recA cells

We studied photoreactivation in cells carrying the multicopy ssb+ plasmid. These cells overproduce single-stranded DNA-binding protein (SSB). Overproduction of SSB enhances the capacity for photoreactivation in recA bacteria but not in the recA+ background. It is suggested that, in recA cells, SSB binds to the dimer region of DNA and that this binding stimulates the process of photoreactivation. In recA+ cells, the same stimulation might be achieved by RecA protein.

PsiB polypeptide prevents activation of RecA protein in Escherichia coli

We further characterize a novel plasmid function preventing SOS induction called Psi (Plasmid SOS Inhibition). We show that Psi function is expressed by psiB, a gene located at coordinate 54.9 of plasmid R6-5 and near oriT, the origin of conjugal transfer. Deletions and amber mutations of the psiB gene permitted us to demonstrate that PsiB polypeptide (apparent molecular weight, 12 kDa) is responsible for Psi function. PsiB protein prevents recA730-promoted mutagenesis and intra-chromosomal recombination but not recombination following conjugation. Overproduction of PsiB protein sensitizes the host cell to UV irradiation. We propose that PsiB polypeptide has an anti-SOS action by inhibiting activation of RecA protein, thus preventing the occurrence of LexA-controlled functions.

Biochemical basis of the temperature-inducible constitutive protease activity of the RecA441 protein of Escherichia coli

We compared the biochemical properties of the RecA441 protein to those of the wild-type RecA protein in an effort to account for the constitutive protease activity observed in recA441 strains. The two RecA proteins have similar properties in the absence of single-stranded DNA binding protein (SSB protein), and the differences that do exist shed little light on the temperature-inducible phenotype observed in recA441 strains. In contrast, several biochemical differences are apparent when the two proteins are compared in the presence of SSB protein, and these are conducive to a hypothesis that explains the temperature-sensitive behavior observed in these strains. We find that both the single-stranded DNA (ssDNA)-dependent ATPase and LexA-protease activities of RecA441 protein are more resistant to inhibition by SSB protein than are the activities of the wild-type protein. Additionally, the RecA441 protein is more capable of using ssDNA that has been precoated with SSB protein as a substrate for ATPase and protease activities, implying that RecA441 protein is more proficient at displacing SSB protein from ssDNA. The enhanced SSB protein displacement ability of the RecA441 protein is dependent on elevated temperature. These observations are consistent with the hypothesis that the RecA441 protein competes more efficiently with SSB protein for limited ssDNA sites and can be activated to cleave repressors at elevated temperature by displacing SSB protein from the limited ssDNA that occurs naturally in Escherichia coli. Neither the ssDNA binding characteristics of the RecA441 protein nor the rate at which it transfers from one DNA molecule to another provides an explanation for its enhanced activities, leading us to conclude that kinetics of RecA441 protein association with DNA may be responsible for the properties of the RecA441 protein.

Activation of protease-constitutive recA proteins of Escherichia coli by rRNA and tRNA

The RecA proteins of the unusually strong protease-constitutive mutants recA1202 and recA1211 can use RNA in addition to single-stranded DNA (ssDNA) as a cofactor in the cleavage of the LexA repressor in vitro. In the presence of rRNA or tRNA, the effectiveness of these proteins decreased in the order RecA1202 greater than RecA1211 much greater than RecA+, which is also the order of their in vivo constitutive protease activities. The effectiveness of rRNA was comparable to that of ssDNA in the cleavage of the LexA repressor by either mutant protease. Although all the common nucleoside triphosphates can act as positive effectors for LexA cleavage by the two mutant proteins in the presence of ssDNA (W. B. Wang, M. Sassanfar, I. Tessman, J. W. Roberts, and E. S. Tessman, J. Bacteriol. 170:4816-4822, 1988), only dATP, ATP, and ATP-gamma-S were effective in the presence of RNA. Our results explain more fully why certain recA mutants have high constitutive protease activities in vivo.

UmuD mutagenesis protein of Escherichia coli: overproduction, purification, and cleavage by RecA

The mutation rate of Escherichia coli increases approximately 100-fold after treatment with replication-inhibiting agents such as UV light. This enhanced mutation rate requires the action of the UmuD and UmuC proteins, which are induced as part of the SOS response to DNA damage. To initiate a biochemical characterization of the role of these proteins, we have developed a plasmid system that gives efficient expression of the umuD and umuC genes. The umuD and umuC genes were placed under the control of a regulated phage lambda PL promoter and a synthetic ribosome-binding site, and the distance to the UmuD start was adjusted to maximize gene expression. Starting from this overproduction system, we have purified the UmuD protein and studied its interaction with RecA. The SOS response is turned on by the capacity of RecA protein to mediate cleavage of the LexA repressor for SOS-controlled operons. Others have shown that UmuD exhibits sequence homology to LexA around the cleavage site, suggesting a possible cleavage reaction for UmuD. We show that RecA mediates cleavage of UmuD, probably at this site. As with LexA, UmuD also undergoes a self-cleavage reaction. We infer that RecA-mediated cleavage of UmuD is another role for RecA in SOS mutagenesis, probably activating UmuD for its mutagenic function.