Independent expression of acetylglutamate kinase and acetylglutamyl phosphate reductase in Neurospora crassa.

摘要

The complex arg-6 locus of Neurospora crassa encodes acetylglutamate kinase (AGK) and acetylglutamyl phosphate reductase (AGPR), the enzymes catalyzing the second and third reactions of arginine biosynthesis. The arg-6 locus is transcribed as a single mRNA, which is translated into a single polyprotein (ARG-6). Upon translocation into the mitochondria, the ARG-6 polyprotein is cleaved into the mature forms of AGK and AGPR. This genetic organization has been found in four other fungi, Saccharomyces cerevisiae, Candida albicans, Schizosaccharomyces pombe, and Aspergillus fumigatus. Unlike fungal AGKs and AGPRs, all prokaryotic equivalents, as well as those of some higher eukaryotes, appear to be expressed from independent genes. The polyprotein structure appears to be a fungal phenomenon and may play a fungal-specific role. The objective of this dissertation is to study the importance of the genetic organization of the arg-6 locus in terms of protein stability, function, expression, and mitochondrial import.; The results indicate that expression of AGK and AGPR as a polyprotein does not appear to be essential for the survival of the organism, the function, stability, or mitochondrial import of either protein. Independent expression of AGK and AGPR results in ∼50% decrease in the levels of each protein. This decrease in protein level does not appear to be the result of the instability of either protein or a lower expression level. The data suggest that the ARG-6 polyprotein might play a role in either the cytoplasmic stability of the polyprotein precursor or the efficiency of import of its components into the mitochondria. Furthermore, the importance of processing of the ARG-6 polyprotein into the mature forms of AGK and AGPR was also examined. Processing appears to be essential for survival of N. crassa.