Participation of transcriptional and post-transcriptional regulatory mechanisms in the control of arginine metabolism in yeast.

In yeast, as in other organisms, amino acid biosynthetic pathways share a common regulatory control. The manifestation of this control is that derepression of the enzymes belonging to several amino acid biosynthetic pathways follows amino acid starvation or tRNA discharging. The arginine anabolic and catabolic pathways are, in addition, regulated specifically by arginine in opposite ways by common regulators. We have measured the mRNA levels for four genes subject to the general amino acid control: HIS4, ARG3, ARG4 and CPAII and compared them to the corresponding enzyme levels. Similarly we have measured the mRNA levels for two genes subject to the arginine specific regulation: ARG3 and CAR1, the former gene belongs to the arginine anabolic pathway and the latter to the arginine catabolic one. HIS4, ARG4 and CPAII enzyme and messenger amounts are perfectly coordinated in all the conditions of general repression or derepression tested. However, arginine does not reduce the level of the ARG3 mRNA enough to explain the reduction of ornithine carbamoyltransferase activity nor does it increase the level of the CAR1 mRNA enough to explain the extent of induction of arginase. Coordination of enzyme and ARG3 mRNA is achieved only when the specific control is eliminated. The half-lives of the ARG3 and CAR1 messengers are enhanced in mutants leading to constitutive expression of ornithine carbamoyltransferase and arginase. These data suggest that the control that coordinates the synthesis of all the amino acids in the yeast cell operates at the level of transcription while the arginine specific regulatory mechanism seems to operate at a post-transcriptional level.