LIMITED DIFFERENTIAL MESSENGER-RNA INACTIVATION IN THE ATP (UNC) OPERON OF ESCHERICHIA-COLI

Individual subunits of ATP synthase, encoded by the eight genes of the atp operon (atpA through atpH), have been found to be synthesized at a 10-fold range in molar amounts (D. L. Foster and R. H. Fillingame, J . Biol. Chem. 257:2009-2015, 1982; K. von Meyenburg, B. B. Jorgensen, J. Nielsen, F. G. Hansen, and O. Michelsen. Tokai J. Exp. Clin. Med. 7 :23-31, 1982). We have determined the functional half-lives at 30-degr ees-C of mRNAs transcribed from these genes either during constitutive expression in a partial diploid strain or after induced expression fr om a plasmid. Accurate decay kinetics of the relative mRNA levels were determined by monitoring the rates of synthesis of the individual ATP synthase subunits by radioactive pulse labeling at different times af ter blocking transcription initiation with rifampin. The mRNA transcri bed from the atp operon was found to be inactivated about twice as fas t as the bulk mRNA in E. coli. Exceptions are the mRNA from the promot er-proximal atpB gene, which was inactivated about three times as fast as the bulk mRNA, and atpC mRNA, the inactivation rate of which was c omparable to that of the bulk mRNA. These moderate differences in the kinetics of functional decay explain only a minor part of the differen ces in expression levels of the atp genes. We conclude, therefore, tha t the individual atp mRNAs must be translated with widely different ef ficiencies. The present analysis further revealed that mRNA degradatio n is sensitive to heat shock; i.e., after incubation at 39-degrees-C f or 5 min followed by a shift back to 30-degrees-C, the decay rate of t he bulk mRNA was decreased by 30%.