A functional role for some Fugu introns larger than the typical short ones: the example of the gene coding for ribosomal protein S7 and snoRNA U17.

The compact genome of Fugu rubripes, with its very small introns, appears to be particularly suitable to study intron-encoded functions. We have analyzed the Fugu gene for ribosomal protein S7 (formerly S8, see Note), whose Xenopus homolog contains in its introns the coding sequences for the small nucleolar RNA U17. Except for intron length, the organization of the Fugu S7 gene is very similar to that of the Xenopus counterpart. The total length of the Fugu S7 gene is 3930 bp, compared with 12691 bp for Xenopus. This length difference is uniquely due to smaller introns. Although short, the six introns are longer than the approximately 100 bp size of most Fugu introns, as they host U17 RNA coding sequences. While four of the six U17 sequences are 'canonical', the remaining two represent diverged U17 pseudocopies. In fact, microinjection in Xenopus oocytes of in vitro synthesized Fugu transcripts containing the 'canonical' U17f sequence results in efficient production of mature U17 RNA, while injection of a transcript containing the U17 psi b sequence does not.