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...