Showing papers on "Genome editing published in 1993"

6 Fungal and Mitochondrial Nucleases

Abstract: I. INTRODUCTION This chapter deals mainly with new developments in understanding structures, functions, and biological roles of three distinct but related classes of fungal and mitochondrial nucleases: the secreted single-strand-specific endonucleases, the intracellular endo-exonucleases, and the major mitochondrial nucleases. These enzymes act on both DNA and RNA to release 5′-phosphoryl or 3′-phosphoryl-terminated products. The extracellular single-strand endonucleases act in conjunction with nucleotide-metabolizing enzymes to scavenge phosphate and nucleosides for cell growth. Endo-exonucleases in nuclei likely have roles in DNA repair, recombination, and possibly DNA replication. The major mitochondrial nucleases, both endo-exonucleases and endonucleases, probably have roles in DNA repair and replication of mitochondrial DNA (mtDNA). In addition, the endo-exonucleases of fungi may also play a role in recombination of mtDNA. On the other hand, the intracellular single-strand-specific endonuclease isolated previously from Neurospora is probably derived from endo-exonucleases via limited proteolysis. Many of the sugar nonspecific endonucleases isolated earlier from mammalian cells may also have been derived in like manner or from mitochondrial endonuclease. Finally, the endonucleases isolated from mammalian mitochondria have been found to be directly related to the endo-exonucleases of fungal mitochondria. II. EXTRACELLULAR (SECRETED) FUNGAL NUCLEASES A. Nucleases S1 and P1 of Aspergillus oryzae and Penicillium citrinum Much of the previous interest in the single-strand-specific endonucleases stemmed from their useful applications in recognizing and cleaving or in eliminating single-strand regions in DNA duplexes or DNA-RNA hybrids (Shishido and Ando 1982). With the recent publications of the amino acid sequences of nuclease S1 (Iwamatsu et al. 1991) and nuclease...

5 The Nucleases of Genetic Recombination

Abstract: I. INTRODUCTION Genetic recombination plays two fundamental roles in the cell: It provides (1) a mechanism for the generation of genetic diversity and (2) a route for the repair of DNA lesions caused by irradiation or chemical damage. Nucleases play important roles in genetic recombination, and this chapter focuses on those enzymes that are directly implicated in the process. Although recombination can take two forms, generalized and site-specific, this chapter concentrates on general genetic recombination, i.e., recombination processes that occur between two homologous chromosomes. For excellent reviews on the mechanisms and enzymes of site-specific and transpositional recombination, see Craig (1988), Cox (1988), Hatfull and Grindley (1988), Landy (1989), and Mizuuchi (1992). Nucleases are required both for the initiation of crossover events between two interacting chromosomes and for the resolution of crossovers to allow the separation of recombinant DNA molecules. Because of the difficulty in directly ascribing many eukaryotic nucleases to a defined role in recombination (due to the lack of mutants), most of our knowledge has come from studies of bacterial and bacteriophage nucleases. However, even in bacteria, the situation is not totally clear, since mutants frequently have little or no phenotype due to the ability of one nuclease to take over in the absence of another. In this case, recombination-defective phenotypes are only observed when two or more nucleases of a given type have been inactivated by mutation. It is possible that the observed redundancy reflects the important role that nucleases play in the recombination process. II. RECOMBINATION MODELS...