Plasmids from Eight Incompatibility Groups Require Similar Host Dna Repair Functions for UV-Protection and Mutagenesis

An assessment of the importance of error-prone repair and point mutations to forward mutation to L-azetidine-2-carboxylic acid resistance in Escherichia coli

By comparison of E. coli WP2 with CM891 (uvrA- pKM101) we found that pKM101 plasmid and uvrA- mutation considerably enhanced both spontaneous and chemically-induced reversion at the trp locus. However, little or no increase was observed for forward mutation at the A2C locus. Furthermore, mutation frequency decline was considerably greater for trp reversion than for mutation to A2Cr. Thus neither error-prone repair nor point mutation seemed likely to be the major mechanism for forward mutation at the A2C locus. Results for spontaneous mutation of recA-, polA- and gyrA- strains showed that polA- and gyrA- gave good increases in forward mutation but not in reversion. It was inferred that deletion, transposition and/or larger chromosomal effects rather than point mutation were mainly responsible for most forward mutation.

Comparison of the Escherichia coli umu+-encoded function with plasmid R46-mediated error-prone repair in DNA-damaged cells

Escherichia coli strain TK701 umu+ was more resistant than strain TK702 umu when tested against bleomycin (BLM), cis-platinum(II) diamminodichloride (PDD), ultraviolet light and methyl methanesulphonate (MMS), which produce single-strand DNA damage. However, the umu mutant was no more sensitive to mitomycin C (MTC) or proflavine (PF), which cause double-strand DNA binding. Strain TK702 umu was nonmutable by any of the agents, whereas mutations were induced in the wild-type strain by PDD, UV, MMS and MTC. The E. coli umu+ function therefore mimics plasmid R46-mediated error-prone repair in protecting only against single-strand DNA damage, whilst enhancing mutagenesis by both single- and double-strand damaging agents. Comparison of plasmid R46-mediated protection and mutagenesis in umu+ and umu strains indicated that the plasmid confers a greater error-prone DNA-repair activity in the mutant. Results are discussed in terms of analogy between host umu+ and plasmid muc+ functions.

Plasmid R46 fails to protect Escherichia coli against double-strand DNA-binding agents but increases their mutagenic activities

Plasmid R46 was tested for its ability to increase survival and mutagenesis of Escherichia coli strain AB1157 following exposure to agents that produce a variety of structural defects in DNA. The plasmid enhanced the mutagenic activities of adriamycin (ADM), N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU), bleomycin (BLM), methyl methanesulphonate (MMS), mitomycin C (MTC), nitrofurazone (NFZ), 4-nitroquinoline-N-oxide (NQO), cis-platinum (II) diaminodichloride (PDD) and proflavine (PF). Furthermore, R46, which is known to protect against BLM and MMS, increased the survival of strain AB1157 following exposure to NFZ, NQO or PDD but not ADM, BCNU, MTC or PF. In fact the plasmid appeared to slightly sensitise its host to the latter drugs. The results are discussed in terms of the nature of the DNA damage produced by these inhibitors and the possible role of plasmid-encoded products in the repair of DNA.