Effect of plasmid pKM101 in ultraviolet irradiated uvr+ and uvr- Escherichia coli

A non-excision uvr-dependent DNA repair pathway of Escherichia coli (involvement of stress proteins)

In UV-irradiated excision-proficient (uvr+) Escherichia coli, pre-induced by simultaneous pre-starvation for thymine (T) and amino acids (AAs), and/or a low UV pre-dose applied after prestarvation for AAs, pyrimidine dimer excision (PDE) is reduced without an adequate increase of UV sensitivity and UV mutagenesis. The unexcised lesions are tolerated by a putative repair pathway that is uvr dependent but does not involve excision. The process consists of PDE inhibition, which requires outer membrane protease OmpT, and subsequent pyrimidine dimer (PD) toleration, which may be mediated by interaction with a sister duplex using a number of SOS and stress-inducible proteins.

Inducible stable DNA replication of Escherichia coli tolerates unexcised pyrimidine dimers in an uvr-dependent manner

Damage-inducible DNA replication (iSDR) was followed in UV-irradiated E. coli uvr+ and uvr B5 cells. Owing to the inhibition of dimer excision in the former (caused by the metabolic treatment), both contained similar amounts of unexcised dimers. Since the iSDR took place in uvr+ but not in uvr B5 cells, it is concluded that the uvr system can tolerate unexcised dimers through the recombinogenic iSDR.

Inhibition of dimer excision in repeatedly UV-irradiated Escherichia coli: its requirement for RecA protein and de novo protein synthesis

In UV-irradiated Escherichia coli dimer excision was found to be inhibited by predamage (M. Sedliaková, F. Masek and J. Brozmanová, FEBS Lett., 23 (1972) 325-326) or overproduction of RecA protein, which suggests that the coating of the dimers by this protein may make them inaccessible to the excision nuclease (M. Sedliaková, K. Kleibl and F. Masek, Mutat. Res., 191 (1987) 13-16). We measured the levels of RecA protein and dimer excision in cells irradiated with (i) a single dose of 50 J m-2, (ii) two separate doses of 30 and 50 J m-2, post-incubated with chloramphenicol; (iii) two separate doses of 30 and 50 J m-2, post-incubated without chloramphenicol. Dimer excision was complete in the first two cases, but in the latter it was inhibited by 40%. At the time of active dimer excision, there were marked differences in RecA protein content between the cells irradiated with a single dose and cells irradiated with two separate doses (both post-incubated without chloramphenicol), which might account for the differences in dimer excision. However, relatively small differences in RecA protein content were found in cells irradiated with two doses and post-incubated with or without chloramphenicol, which could therefore not account for the differences in dimer excision. The data suggest that the inhibition of dimer excision involves some short-lived component(s) other than RecA protein.

UmuC product contributes to the inhibition of dimer excision produced by thymine-less-amino acid-less pretreatment in UV-irradiated Escherichia coli

In UV-irradiated Escherichia coli, predamaged by thymine-amino acid starvation or a UV predose, a large amount of dimers may remain unexcised and may be tolerated by an error-free mechanism, which requires the function of uvr, recA and lexA genes. A possible role of the umuC gene in both the inhibition of dimer excision and the toleration of unexcised dimers is investigated. Data suggest that the UmuC gene product is not absolutely necessary for the inhibition of dimer excision in UV-irradiated thymine-less-amino acid-less pretreated cells, but that it contributes to it. However, the UmuC product does not seem to be involved in the toleration of unexcised dimers which is dependent on uvr, recA and lexA.