Figure 16.1. Schematic representation of three different situations with internal initiation on polycistronic mRNAs in Prokaryotes.

A: Independent initiation on each cistron.

B: Initiation on a downstream cistron induced by translation of the preceding cistron. Presumably the translation of the upstream cistron disrupts a long-range interaction within mRNA and thus opens the RBS of the downstream cistron. C: Reinitiation. Downstream cistrons are incapable of initiating with free 30S subunits. Instead, the 30S subunits after termination of translation of the preceding cistron slide phaselessly along mRNA and reinitiate at a nearby RBS, that is the neighbour downstream cistron.

(L10) cistron; the initiation at the RBS by free ribosomes is impossible. When ribosomes initiate at the rplJ cistron and then translate the message, they melt this structure and open the rplL RBS. The strength of the open RBS is found to be much higher than that of the rplJ RBS providing a very efficient binding of free ribosomes and thus a high level of initiation. As a result, the translation of the rplL cistron yields four times more copies of protein L7/L12 than the number of copies of protein L10 produced from the preceding rplJ message:

rplJ rplL

L10 4xL7/L12

The proportion 4:1 corresponds to the molar ratio of protein L7/L12 to protein L10 in the ribosome.

The rearrangement of a secondary structure resulting in an appearance of an open RBS can be also induced by ribosomes stalled at a specific position of a preceding coding sequence rather than by actively translating ribosomes (reviewed by de Smit & van Duin, 1990). This mechanism is used in several cases for regulation of the synthesis of some antibiotic-resistance factors. For example, translation of cat mRNA encoding for chloramphenicol acetyltransferase (enzyme which inactivates chloramphenicol) in Grampositive bacteria is induced by chloramphenicol in the following way. The cat mRNA is the second cistron in a bicistronic message and has a hidden RBS involved in the formation of a stable hairpin together with the end of the preceding cistron (Fig. 16.2). The preceding cistron encodes for a short peptide and has an open RBS. In the absence of chloramphenicol ribosomes translate the first cistron but cannot translate the second (cat) cistron: independent initiation is not allowed, and the translation of the preceding cistron till start leader

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