RNA Cleavage Linked with Ribosomal Action

Ribonuclease LS in Escherichia coli is an antagonist of bacteriophage T4. This RNase efficiently cleaves T4 mRNAs and leads to the silencing of late genes, thus blocking T4 growth. We previously found that, when two consecutive ochre codons were placed in the open reading frame of T4 soc, RNase LS cleaved soc mRNA at a specific site downstream of the ochre codons. Here, we demonstrate that RNase LS cleaves soc RNA at the same site even when only a single ochre codon is present or it is replaced with either an amber or an opal codon. On the other hand, disruption of the Shine-Dalgarno sequence, a ribosome-binding site required for the initiation of translation, eliminates the cleavage. These results strongly suggest that RNase LS cleaves in a manner dependent on translation termination. Consistent with this suggestion, the cleavage dependency on an amber codon was considerably reduced in the presence of amber codon-suppressing tRNA. Instead, two other cleavages that depend on translation of the region containing the target sites occurred further downstream. Additional analysis suggests that an interaction of ribosome with a stop codon might affect the selectivity of RNase LS over a distance in an mRNA molecule. This effect of ribosome could be possible by remodeling of high-order structure of the mRNA molecule.

Key Words: E. coli, RNA cleavage, RNase LS, ribosome, termination codon

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