The stability of the Chlorella nitrate reductase mRNA is determined by the secondary structure of the 5'-UTR: implications for posttranscriptional regulation of nitrate reductase.

Nitrate reductase (NR; EC 1.6.1.1-3) can be controlled at both transcriptional and posttranscriptional levels. Here we describe stability of NR mRNA as a mechanism of control. The NR gene in Chlorellavulgaris (Warburg strain) transcribes a stable mRNA and an unstable mRNA. In-vitro-synthesized transcripts representing these mRNAs show the same stability characteristics. The unstable mRNA is 30 nucleotides longer at the 5′-UTR compared to the stable mRNA. Using an RNA-folding program the 5′-UTR of the longer unstable RNA showed a more extensive stem-loop structure compared to the more linear form of the shorter stable mRNA. Transcripts representing RNAs with intermediate 5′-UTRs folded similarly to the long form and were unstable, or similarly to the short form and were more stable. Thus the secondary structure of the 5′-UTR of NR mRNA is important in the stability of NR transcripts in Chlorella and allows the cell to respond to changes in nitrogen source in an energy-efficient manner.