A competition mechanism regulates the translation of the Escherichia coli operon encoding ribosomal proteins L35 and L20

Escherichia coli ribosomal protein (r-protein) L20 is essential for the assembly of the 50S ribosomal subunit and is also a translational regulator of its own rpmI-rplT operon, encoding r-proteins L35 and L20 in that order. L20 directly represses the translation of the first cistron and, through translational coupling, that of its own gene. The translational operator of the operon is 450 nt in length and includes a long-range pseudoknot interaction between two RNA sequences separated by 280 nt. L20 has the potential to bind both to this pseudoknot and to an irregular hairpin, although only one site is occupied at a time during regulation. This work shows that the rpmI-rplT operon is regulated by competition between L20 and the ribosome for binding to mRNA in vitro and in vivo. Detailed studies on the regulatory mechanisms of r-protein synthesis have only been performed on the rpsO gene, regulated by r-protein S15, and on the alpha operon, regulated by S4. Both are thought to be controlled by a trapping mechanism, whereby the 30S ribosomal subunit, the mRNA, and the initiator tRNA are blocked as a nonfunctional preternary complex. This alternative mode of regulation of the rpmI-rplT operon raises the possibility that control is kinetically and not thermodynamically limited in this case. We show that the pseudoknot, which is known to be essential for L20 binding and regulation, also enhances 30S binding to mRNA as if this structure is specifically recognised by the ribosome.