Regulation of CPSase, ACTase, and OCTase genes in Medicago truncatula: Implications for carbamoylphosphate synthesis and allocation to pyrimidine and arginine de novo biosynthesis.

In most prokaryotes and many eukaryotes, synthesis of carbamoylphosphate (CP) by carbamoylphosphate synthetase (CPSase; E.C. 6.3.5.5) and its allocation to either pyrimidine or arginine biosynthesis are highly controlled processes. Regulation at the transcriptional level occurs at either CPSase genes or the downstream genes encoding aspartate carbamoyltransferase (E.C. 2.1.3.2) or ornithine carbamoyltransferase (E.C. 2.1.3.3). Given the importance of pyrimidine and arginine biosynthesis, our lack of basic knowledge regarding genetic regulation of these processes in plants is a striking omission. Transcripts encoding two CPSase small subunits (MtCPSs1 and MtCPSs2), a single CPSase large subunit (MtCPSl), ACTase (MtPyrB), and OCTase (MtArgF) were characterized in the model legume Medicago truncatula. Quantitative real-time PCR data provided evidence (i) that the accumulation of all CPSase gene transcripts, as well as the MtPyrB transcript, was dramatically reduced following seedling incubation with uridine; (ii) exogenously supplied arginine down regulated only MtArgF; and (iii) mRNA levels of both CPSase small subunits, MtPyrB, and MtArgF were significantly increased after supplying plants with ornithine alone or in combination with uridine or arginine compared to plants treated with only uridine or arginine, respectively (P< or =0.05). A proposed novel, yet simple regulatory scheme employed by M. truncatula more closely resembles a prokaryotic control strategy than those used by other eukaryotes.