THE GENETIC CONTROL OF ADENYLOSUCCINASE IN Neurospora Crassa

Abstract

Combined genetic and biochemical studies were performed on a series of adenine-specific mutants in N. crassa. Twenty-one mutants of independent origin have arisen at a single locus (ad-4) in linkage group III between 2 closely linked marker genes. Adenine-independent isolates arise with a low frequency in some crosses of these mutants, and the majority of these types are pseudo-wilds, although true wild types occur in certain combinations. The recombination mechanism associated with the origin of wild types has not yet been established. Biochemically, all the mutants are blocked in the terminal step in adenine biosynthesis, involving the splitting of adenosine monophosphate succinate (AMP-S) to adenosine monophosphate (AMP), and lack (or have impaired activity for) the AMP-S splitting enzyme, adenylosuccinase. One of the mutants is temperature-sensitive and produces an adenylosuccinase which in crude extracts is much more thermolabile than that from wild type. Certain of the ad-4 mutants are capable of reverse mutation to adenine-independent phenotypes, and these possess restored adenylosuccinase activity, although at levels (or of stabilities) below that of wild type. Thus, in this instance, forward mutation at a single locus in the wild type to adenine requirement results in the loss of activity of a specific enzyme involved in adenine biosynthesis, and reverse mutation to adenine independence results in the restoration of this activity. Additionally, changes at this locus, arising from either forward or reverse mutational events, may produce diverse mutant and revertant types as judged by qualitative and quantitative tests of enzyme activity. Certain combinations of ad-4 mutants form heterocaryons (bicaryons) able to grow in the absence of adenine. Although the individual mutants lack detectable adenylosuccinase activity, this enzyme is synthesized by the bicaryons. Possible mechanisms for this unexpected type of complementation between alleles in enzyme formation are discussed, along with the implications of such complementation for biochemical genetic theory in general.