Regulation of α operon gene expression in Escherichia coli: A novel form of translational coupling

Abstract

The α operon of Escherichia coli contains the genes for ribosomal proteins S13, S11, S4, RNA polymerase subunit α, and r-protein L17, in this order. Previous studies have shown that translation of all four ribosomal proteins is regulated by S4, and that binding of S4 to the mRNA at the start site for S13 translation is probably responsible for the regulation of translation of S13, S11 and S4. The α gene is “unique” in that it is located between the genes for two ribosomal proteins (S4 and L17) and yet appears to be regulated independently of them. In the present studies, we have measured the synthesis rates of all the α operon proteins under a variety of physiological conditions. Our results confirm that α gene expression is regulated independently of the co-transcribed ribosomal protein genes and is relatively insensitive to translational feedback repression by S4. S 1 nuclease analysis of α operon mRNA failed to reveal the presence of any unique transcription start or mRNA cleavage that leads to separation of the α cistron from preceding ribosomal protein cistrons. Therefore, it appears that differential regulation of α synthesis takes place at the level of mRNA translation. We have also carried out a deletion analysis of the α operon leader and identified a region of the α operon leader mRNA that is required for regulation by S4. Furthermore, deletion of this region results in increased synthesis of L17 together with S13, S11 and S4, whereas α synthesis did not increase significantly. Therefore, we conclude that interaction of S4 with this single target site results in translational repression of not only the proximal three cistrons for S13, S11 and S4 but also that of the last cistron, L17, without affecting the intervening α cistron.