C. elegans codes for two classes of SL RNA, SL1 RNA and SL2 RNA, that donate their 5'' 22 nt to mRNAs in trans-splicing. A two-polypeptide complex, SNA-2/SNA-1, unique to nematodes, is bound to the SL1 RNA but not to SL2 RNA. Surprisingly however, a different class of snRNA, whose function is unknown, also unique to nematodes, Sm Y RNA, contains a SNA-2/SUT-1 complex, where SUT-1 is a paralog of SNA-1. Deletions of
sna-1 or
sut-1 are viable, but 15 deg sterile. They can also phenocopy a variety of morphological abnormalities at 15 deg . Both the
sna-2 deletion and the
sna-1/sut-1 double deletion are lethal. Thus SNA-1 and SUT-1 are functionally redundant, even though they are bound to different classes of snRNP. Why does SL2 RNA have neither of these proteins? How can these results be rationalized? The answer may lie in the issue of how the Sm ring proteins are recycled after trans-splicing. SL1, SL2 and Sm Y snRNPs all contain the Sm proteins, which are normally components of stable spliceosomal snRNPs, U1, U2 etc. We think the SNA and SUT-1 proteins may collaborate with Sm Y RNA to recycle the Sm proteins after splicing. The Evans lab showed that Sm proteins are present in P granules and are required for fertility. We hypothesize that at 15 deg , when either SUT-1 or SNA-1 is missing, there is a failure to recycle some of the Sm protein used in trans-splicing, which could result in splicing problems, producing morphological defects, or failure to make active P granules, producing sterile worms. In support of this idea, we have found a strong genetic interaction between
prmt-5, whose protein product modifies Sm proteins, and both
sut-1 and
sna-1: whereas the
prmt-5 deletion alone is without detectable phenotype, this mutation suppresses the
sut-1 and enhances the
sna-1 deletion phenotypes. Interestingly, these genes also interact with mutations in genes encoding P granule proteins,
pgl-1 and
glh-1.