Opening the closed ribosome-binding site of the lysis cistron of bacteriophage MS2.

In prokaryotes gene expression is mainly regulated at the levels of transcription and translation. An important form of translational control operates at the initiation of protein synthesis. For instance the translation of an existing mRNA can be prevented by features in the mRNA structure that prohibit binding of ribosomes. This type of control is frequently applied to polycistronic mRNA to forestall translation of a downstream cistron until the 5' neighbouring cistron has been read (1-7). Such translational coupling or sequential reading also facilitates the shutting off of several cistrons from one control point(6). An interesting example of a nontranslatable message is the lysis (L) cistron, present as an overlapping gene in the RNA bacteriophage MS2 (refs 8-10; Fig. 1). The start of the L cistron is not directly accessible to ribosomes. Instead its translation is strictly coupled to the passage of ribosomes over the preceding coat cistron(2). We have now analysed which features in the MS2 RNA structure deny ribosomes access to the start of the L message. We report here that small deletions, introduced about 40 nucleotides 5' to the start codon of the L gene, remove the initiation barrier and open the cistron to independent translation. An RNA secondary structure accounting for the closed state of the ribosome binding site is proposed.