Endonucleolytic cleavage of a maternal homeo box mRNA in Xenopus oocytes.

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Abstract
We have identified a messenger RNA (mRNA) sequence from a Xenopus homeo box-containing gene that is the target for a sequence-specific endoribonuclease in vivo. Synthetic RNA transcribed from an allele of the maternal gene Xlhbox2B is efficiently cleaved when injected into Xenopus oocytes. The cleavage sequence lies between the protein-coding region and a 600-base 3'-untranslated region. Intermediates in degradation are readily observed: Both the 5' and 3' products of cleavage are recovered, thus showing that the cleavage activity is an endonuclease. When a 90-base region of the Xlhbox2B sequence is inserted into a second homeo box RNA that is normally stable, it is sufficient to confer an identical cleavage reaction on the hybrid RNA. The cleaved region contains a repeated sequence motif and is cut at multiple sites. Inhibition of translation does not affect the rate or extent of cleavage, while the coinjection of antisense RNA complementary to the 90-base region completely blocks the reaction. Because most mRNAs are not found on polysomes during oogenesis, translation-independent cleavage at such sites may provide a novel post-transcriptional mechanism to regulate the amount of mRNA available for embryogenesis.