Transcriptional regulation of cytochrome d in nitrogen-fixing Azotobacter vinelandii. Evidence that up-regulation during N2 fixation is independent of nifA but dependent on ntrA.

Cytochrome d has been postulated to be the "respiratory protection" oxidase of Azotobacter vinelandii, allowing this organism to fix nitrogen under aerobic growth conditions. We have previously cloned and characterized the structural genes for the A. vinelandii cytochrome d (cydA and cydB). The cyd genes are co-transcribed, yielding an mRNA of approximately 3.6 kilobase pairs. The level of the cyd message was 2-3-fold higher in cells that were fixing nitrogen, as compared with non-nitrogen-fixing cells. RNase protection analysis was used to determine the transcriptional start site at 275 bases upstream of the initiator ATG of cydA, and this start site was the same for nitrogen-fixing and non-nitrogen-fixing cells. The cyd promoter has sequence similarities to the canonical Escherichia coli promoters, which are transcribed by the major sigma 70 form of RNA polymerase. Plasmid-borne lacZ transcriptional fusions were constructed, using approximately 650 base pairs of 5'-upstream sequences of the cyd structural genes. This region had a strong promoter activity which was further up-regulated 1.5-2.5-fold upon the induction of nitrogen fixation. The cyd-lacZ fusions were characterized in a nifA- as well as an ntrA- background. Mutations in neither of these nif regulatory genes affected the constitutive expression of cyd under non-nitrogen-fixing conditions. However, the up-regulation of this promoter during the induction of nitrogen fixation was abolished only in the ntrA- background. Based on these results, the cytochrome d promoter of A. vinelandii belongs to a new class of nitrogen-regulated promoters which, unlike the authentic nif genes, does not require the ntrA gene product for its expression. The up-regulation of this promoter during nitrogen fixation, however, requires the ntrA gene product.

Pre-translational regulation of the (Na+ + K+)-ATPase in response to demand for ion transport in cultured chicken skeletal muscle.

The expression of the (Na+ + K+)-ATPase in cultured chicken skeletal muscle can be altered by varying the demand for ion transport. Veratridine, an activator of voltage-sensitive Na+ channels, causes a specific transient increase in biosynthesis of the sodium pump that accounts for the doubling of the number of (Na+ + K+)-ATPase molecules in the sarcolemma (Wolitzky, B.A., and Fambrough, D.M. (1986) J. Biol. Chem. 261, 9990-9999). Here we report a study of veratridine-induced up-regulation, focusing upon alpha- and beta-mRNA levels and transcription rates. Myotubes normally express the alpha 1-isoform mRNA and the beta-mRNA at a molar ratio of 0.6 +/- 0.1 (S.D.). In the presence of veratridine, the beta-mRNA is transiently up-regulated approximately 3-fold. The kinetics of this increase parallel the rate of beta-subunit protein synthesis. The increase in beta-mRNA during up-regulation is accomplished by an increase in the transcription rate of the beta gene. The veratridine-induced increase in beta-mRNA is not blocked by cycloheximide. The alpha-mRNA also increases during exposure to veratridine, but this increase is very modest and occurs very late in the up-regulation process. The increased beta-mRNA results in over-production of beta-subunits, which we postulate drives more efficient assembly of alpha beta complexes, i.e. sodium pump molecules. As the up-regulated state is achieved the level of beta-mRNA falls abruptly, reflecting a marked decrease in beta-mRNA stability. Treatment of up-regulated myotubes with tetrodotoxin, a veratridine antagonist, results in rapid down-regulation of the sodium pump, while having little or no effect on the levels of alpha 1- and beta-mRNAs.