THE PROMOTER-PROXIMAL, UNSTABLE IB REGION OF THE ATP MESSENGER-RNA OFESCHERICHIA-COLI - AN INDEPENDENTLY DEGRADED REGION THAT CAN ACT AS ADESTABILIZING ELEMENT

Differential expression of the genes in the Escherichia coli atp (unc) operon is achieved via control of the translational initiation, trans lational coupling and mRNA stability of the respective genes. The atp1 B region of the polycistronic mRNA is less stable than the remaining s even genes. We have investigated the functional half-lives of thr atp genes in reconstructed versions of the operon. In order to be able to do this reliably, we have readdressed the interpretation of the comple x functional inactivation data obtained by means of transcriptional in hibition using rifampicin. Our results indicate the usable information to be gleaned from this commonly applied technique, while identifying the potential errors in their quantitative interpretation. We estimat e that the functional half-life of atpB is slightly over one-half that of atpE and the other atp genes, while atp1 is at least two times les s stable than atpB. The instability of the atp1 mRNA was also demonstr ated by its rapid fragmentation. Relocation of atplB to a position in the promoter-distal region of the operon between atpG and atpD did not change the inactivation rate of atpB. However, it did destabilize the atpG mRNA. Examination of the physical degradation of atp1 mRNA shows particularly rapid cleavage in this gene, thus explaining the destabi lization effect. The atplB segment is therefore an autonomously unstab le region that can act as a destabilizing element for upstream-located genes in a polycistronic environment.