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Genetic control of the UV-induced SOS mutator effect in single- and double-stranded DNA phages
G Maenhaut-Michel1, P Caillet-Fauquet
1Department of Molecular Biology, Faculty of Sciences, Université Libre de Bruxelles, Rhode-St-Genèse, Belgium.
Abstract:
The SOS hypothesis postulated that the mutator effect on undamaged DNA that generates phage-untargeted mutagenesis (UTM) results directly from the mechanism of targeted mutagenesis. RecA protein, which stimulates the cleavage of both the LexA repressor and UmuD protein, and the UmuDC gene products are required for UV-induced targeted mutagenesis. The use of phage lambda for analyzing UV-induced mutagenesis has permitted a distinction to be made between the mechanisms of targeted and untargeted mutagenesis, in that the two processes differ with respect to their genetic requirements for recA+ and umuDC+ genes. In this paper, we show that (i) proficiency for excision repair is required for UTM in double-stranded DNA phage but not in single-stranded DNA phage; (ii) the umuC function, which is not required for UTM of the double-stranded DNA phage lambda, is necessary for untargeted mutagenesis of the single-stranded DNA phages M13 and phi X174; (iii) for both single-stranded and double-stranded DNA phage, UV irradiation of the host increases the level of recA730-induced UTM. Our results are also consistent with the interpretation that the expression of untargeted mutagenesis in phage lambda and in M13 depends on the polymerase and to a lesser extent on the exonuclease 5'----3', activities of PolI. These results suggest that the involvement of the RecA and UmuDC proteins may be related to more than the presence of base damage in the DNA substrate.
Insights
Untargeted mutagenesis (UTM) in bacteriophages is influenced by DNA type and host cell repair mechanisms. RecA and UmuDC proteins play roles beyond simple DNA damage response.
Area of Science:
- Molecular Biology
- Genetics
- Microbiology
Background:
- The SOS hypothesis suggests untargeted mutagenesis (UTM) arises from targeted mutagenesis mechanisms.
- RecA protein and UmuDC gene products are essential for UV-induced targeted mutagenesis.
- Phage lambda studies differentiate targeted and untargeted mutagenesis based on genetic requirements.
Purpose of the Study:
- To investigate the genetic requirements for untargeted mutagenesis (UTM) in different bacteriophages.
- To clarify the roles of RecA, UmuDC, and host repair pathways in UTM.
- To explore the involvement of DNA polymerase I (PolI) in UTM.
Main Methods:
- Comparative analysis of UV-induced mutagenesis in double-stranded (phage lambda) and single-stranded (M13, phi X174) DNA phages.
- Assessment of the requirement for excision repair proficiency in UTM.
- Evaluation of the necessity of umuC function for UTM in different phage types.
- Investigation of the impact of host UV irradiation on recA730-induced UTM.
- Analysis of DNA polymerase I (PolI) activities in UTM.
Main Results:
- Excision repair is required for UTM in double-stranded DNA phages but not single-stranded DNA phages.
- The umuC function is essential for UTM in single-stranded phages (M13, phi X174) but not for double-stranded phage lambda.
- Host UV irradiation enhances recA730-induced UTM for both single-stranded and double-stranded DNA phages.
- UTM in phage lambda and M13 involves DNA polymerase I (PolI) polymerase and exonuclease activities.
Conclusions:
- The genetic requirements for UTM differ between single-stranded and double-stranded DNA phages.
- RecA and UmuDC proteins are involved in UTM through mechanisms potentially extending beyond direct DNA damage response.
- DNA polymerase I plays a role in the expression of UTM in certain phage systems.