Depression of enzyme synthesis in response to arginine limitation in Neurospora crassa

Ornithine carbamoyltransferase and argininosuccinase, two enzymes involved in arginine synthesis, are regulated by cross-pathway amino acid control in Neurospora and show derepression in response to limitation of any one of a number of amino acids. The effects of varying the severity of arginine limitation upon the synthesis of these enzymes, in mycelial cultures of an arginine auxotrophic strain, are reported here. Depression occurred at arginine concentrations sufficient to allow normal rates of protein accumulation, leading to increases of not more than fourfold in the absolute rate of enzyme synthesis. On the other hand, differential rates of enzyme synthesis increased progressively up to 20-fold or more under extreme conditions of arginine limitation that also limit net protein synthesis. The major part of the derepression response thus occurred at arginine concentrations that allowed low net rates of protein synthesis. The physiological significance of this is not yet understood. Our evidence suggests that these responses were mediated entirely through the cross-pathway control system, and may not be untypical (allowing for variations in magnitude) of depression resulting through this mechanism in Neurospora.