GENETIC STUDIES OF NITRATE ASSIMILATION IN ASPERGILLUS NIDULANS

Abstract

Summary(1) In Aspergillus nidulans, at least 16 genes can mutate to affect the reduction of nitrate to ammonium, a process requiring two enzymes, nitrate reductase and nitrite reductase.(2) niaD is the only gene whose effects on enzyme structure are confined to nitrate reductase alone. It specifies a core polypeptide, one or more of which form the basic subunit of nitrate reductase, molecular weight 50000.(3) At least five cnx genes together specify a molybdenum co‐factor, necessary for the activity of nitrate reductase, and of xanthine dehydrogenases I and II. The cnxH gene specifies a polypeptide component of this co‐factor, and the cnxE and F gene products are involved in co‐factor elaboration, The role of the remaining cnx genes is at present unknown.(4) Functional nitrate reductase has a molecular weight of 200000 and is likely to consist of four subunits, together with one or more molecules of the cnx‐specified co‐factor.(5) The co‐factor plays a catalytic role in the aggregation of nitrate‐reductase subunits.(6) The niiA gene is the structural gene for nitrite reductase.(7) Other genes affecting nitrate assimilation are either regulatory or bring about their effects indirectly.(8) Of the genes affecting nitrate assimilation, close linkage is found only between the niiA and niaD genes.(9) Nitrate and nitrite reductases are subject to control by nitrate induction and ammonium repression.(10) Nitrate induction is mediated by the nirA gene whose product must be active for the niiA and niaD genes to be expressed. Since most niaD mutants produce nitrite reductase constitutively, it is likely that the nirA gene product is normally inactivated by nitrate reductase, but only when the latter is not complexed with nitrate,(11) Ammonium repression is mediated by the areA gene, whose product must be active for the expression of the niiA and niaD genes, and which is inactive in the presence of ammonium.(12) The tamA gene may function similarly to the areA gene, both gene products being necessary for the expression of the niiA and niaD genes.(13) Although the niiA and niiD genes are probably contiguous, they are not likely to be organized into a structure equivalent to a bacterial operon.(14) Whereas the areA and nirA genes regulate the synthesis of nitrate and nitrite reductases, it is probable that at least nitrate reductase is also subject to post‐translational control, the presence of active enzyme being correlated with high levels of NADPH.(15) The regulation of the pentose‐phosphate pathway, of mannitol‐I‐phosphate dehydrogenase and of certain activities required for the catabolism of some nitrogen‐containing compounds appears to be connected with that of nitrate assimilation. In all cases, it is probable that the nirA gene and nitrate reductase itself are involved.