CHLAMYDOMONAS U2, U4 AND U6 SNRNAS - AN EVOLUTIONARY CONSERVED PUTATIVE 3RD INTERACTION BETWEEN U4 AND U6 SNRNAS WHICH HAS A COUNTERPART INTHE U4(ATAC)-U6(ATAC) SNRNA DUPLEX

The spliceosomal UsnRNAs U2, U4 and U6 from the green alga Chlamydomon as reinhardtii (Cre) were sequenced using a combination of RNA and cDN A sequencing methods and were compared to other sequenced UsnRNAs. The lengths of Cre U6 and Cre U2 RNAs are similar to those of their highe r plant equivalents. Cre U4 RNA is shorter (139 nt) than its counterpa rt from higher plants (150-154 nt), and contains stem IV and loop D wh ich are absent, with the exception of the Tetrahymena U4 RNA, from the U4 RNAs of other unicellular organisms studied to date. Base-pairing interactions between U6 and U4 RNAs and between U6 and U2 RNAs, identi cal to those described for mammalian and yeast systems, are structural ly feasible in the Cre system. In addition, based on comparative analy ses of the predicted U4/U6 RNA duplex from various species, an evoluti onary conserved third putative U6-U4 interaction was found. Interestin gly, it can also be formed with the recently discovered U6(atac) and U 4(atac) RNAs. This is a strong support in favor of the possible biolog ical significance of this third putative interaction. Based on compara tive analysis, an extension of the earlier described U6-U2 interaction patterns is also proposed.