Degradation of Bacillus subtilis glutamine phosphoribosylpyrophosphate amidotransferase in vivo

Abstract

Glutamine phosphoribosylpyrophosphate amidotransferase, the first enzyme of purine nucleotide biosynthesis, is inactivated and degraded in Bacillus subtilis during carbon, nitrogen, or amino acid starvation. Amidotransferase is stable in exponentially growing cells, and synthesis of the enzyme ceases prior to its inactivation at the end of exponential growth. Inactivation has been previously shown to result from reaction of an essential [4Fe-4S] center with oxygen. In this work, monospecific antibodies against amidotransferase have been used to demonstrate that inactivation is followed by proteolytic degradation in vivo and that the metabolic requirements for degradation differ from those for inactivation. Unlike inactivation, degradation is inhibited by addition of 10 mM KCN or antibiotic inhibitors of RNA and protein synthesis to glucose-starved cells. The cross-reactive material that accumulates when degradation is inhibited by chloramphenicol initially has native subunit molecular weight, but lower molecular weight polypeptides slowly accumulate. Degradation, but not inactivation, of amidotransferase is strongly inhibited during amino acid starvation of a relA strain. Degradation of amidotransferase is inhibited by pseudomonic acid, an antibiotic that blocks protein synthesis but permits a normal stringent response. This result indicates that both protein synthesis and normal relA gene function are required for degradation.