Molecular mechanism of introduction of the hidden break into the 28S rRNA of insects: implication based on structural studies.

We determined the structure around the gap region between 28S alpha and 28S beta rRNA of Bombyx mori, a lepidopteran insect, to know the introduction mechanism of the hidden break, an interruption of the phosphodiester bond specific to the 28S rRNA of protostomes. Sequence analysis and S1 nuclease mapping suggested that a stretch of 30 nucleotides is excised from the mid region of the 28S rRNA to generate the hidden break. The length of excluded stretch was very various among three insects so far studied. However, the gap responsible for the hidden break was located in a fixed position in the 28S rRNA irrespective of insect species. It was suggested that extremely AU-rich sequence including the specific UAAU tract forming a loop can be a signal for the introduction of the hidden break. The same signal seemed also involved in splitting the dipteran 5.8S rRNA.