Possible role for mRNA stability in the ammonium-controlled regulation of nitrate reductase expression.
Ammonium, or a metabolite of ammonium, represses the expression of nitrate reductase (NR) in Chlorella vulgaris. The removal of ammonium and addition of nitrate (induction) resulted in a rapid (20 min) peaked synthesis of NR mRNA. Nitrate reductase protein and activity increased at a much lower rate, reaching their maxima by 8 h. Ammonium added to nitrate-grown cells resulted in a dramatic decrease in NR mRNA from a steady-state level to undetectable levels within 15 min of ammonium addition. Nitrate reductase activity and protein levels decreased to 20% and 40% of initial levels respectively over 8 h. The half-life for NR mRNA under these conditions was estimated to be less than 5 min, compared with 120 min for NR protein. Such rapid decreases in NR mRNA suggested a degradation and/or cessation in NR mRNA transcription. No apparent difference in NR mRNA-specific RNAase activity of crude cell extracts (NR-induced or repressed) was observed. However, a significant difference in the susceptibility to degradation of NR mRNA from long-term nitrate-grown cells compared with the NR mRNA isolated from short-term induced cells (20 min in nitrate) was observed. NR mRNA isolated from long-term-nitrate-grown cells was completely degraded by RNAases in cell extracts under conditions in which the NR mRNA isolated from short-term induced cells was resistant to degradation. These results suggest that mRNA stability may be an important factor in the metabolic regulation of assimilatory nitrate reductase in Chlorella.
Cannons
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USF College of Medicine, Department of Biochemistry and Molecular Biology, Tampa, FL 33612.
Pendleton
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D002708:Chlorella / Q000201:enzymology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000235:genetics* / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology*; D012333:RNA, Messenger / Q000378:metabolism*; D012260:Ribonucleases / Q000378:metabolism
D000644:Quaternary Ammonium Compounds; D012333:RNA, Messenger; D009565:Nitrate Reductases; D050901:Nitrate Reductase; D012260:Ribonucleases