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Chapter 8 : DNA Repair and Recombination
Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
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Investigations using filamentous fungi have made major contributions to the understanding of the core biological processes of DNA repair and recombination, as certain model species provide particularly favorable opportunities for insight. The Ascomycete fungi, including the filamentous fungus Neurospora crassa, conveniently provide a full set of the products of a single meiosis packaged in spores inside a single ascus. The study of DNA repair began with the discovery of photoreactivation and UV-sensitive mutants in bacteria, followed by isolation and characterization of mutants with sensitivity to UV, IR, and chemical mutagens in both Escherichia coli and the yeast S. cerevisiae. The segregation patterns of parental DNA in recombinant chromosomes from yeasts and filamentous fungi as considered stimulated Whitehouse and Holliday to formulate models of meiotic recombination, and aspects of both models remain valid to this day. Research into DNA repair and recombination can now proceed apace by deletion of each annotated gene with a predicted role in these processes, enabling to move beyond understanding individual repair systems to an understanding of more complex networks. Epigenetic control of DNA metabolism undoubtedly plays a part in DNA repair and recombination. The isolation of mutants sensitive to mutagens remains as an indispensable tool if a comprehensive understanding of DNA repair and recombination is developed. The filamentous fungi have greater genetic complexity than yeast and are more similar to complex organisms such as mammals. The filamentous fungi will continue to provide efficient model systems for such investigation.
Pathways (major, thick arrows; minor, narrow dashed arrows) for integration of exogenous DNA into chromosomal DNA. Exogenous DNA is integrated into the chromosomes by two major pathways, mus-11 dependent and mus-52 dependent. The mus-11-dependent pathway has three branches. Two branches yield homologous integration (HI); one is mei-3 dependent, and the other is mei-3 independent. The third mus-11 branch is mus-52 independent but, like the major mus-52-dependent pathway, leads to nonhomologous integration (NHI). Both mus-52- dependent and mus-52-independent NHI pathways require mus-53, mus-56, and mus-57. The genes in parentheses represent S. cerevisiae homologs.
Epistatic and synthetic lethal relationships of mutagen-sensitive mutations in N. crassa. A solid line indicates a synthetic lethal relationship, and a dotted line indicates an epistatic relationship. Arrows indicate which mutation is epistatic.
Models for meiotic recombination (after Baudat and de Massy, 2007 ). The central column depicts the generation of crossovers. All recombination is thought to be initiated by a break in both strands of the duplex (B). The 5’ ends are resected, and one of the 3’ ends invades the homologous duplex (C), where it is extended by DNA polymerase using the homolog as a template (D). The D loop formed by displacement of a strand of the invaded chromosome provides a template for repair of the second strand of the initiating chromosome (E). The ligation of ends forms two Holliday junctions that when resolved yield a crossover (F). Synthesis-dependent strand annealing is depicted on the right branch. Here, instead of ligation leading to paired Holliday junctions, the newly synthesized DNA ends are unraveled from the template to anneal one with the other (E’). The gap is closed and then ligated (F’). Only the initiating chromosome is converted, and there is no crossing-over. The left branch indicates the existence of a pathway yielding noninterfering crossovers, although details of the mechanism remain to be elucidated.
A partial linkage map of Neurospora, showing the locations of the known rec genes and the regions in which each rec gene influences recombination (arrow heads). Spheres represent the centromeres. The map is not to scale, and only part of each chromosome is shown.
DNA repair pathways