Primary and secondary structure of dinoflagellate U5 small nuclear RNA.

U5 RNA is one of the six capped small nuclear RNAs present in most eukaryotic cells. Like U1, U2, U4 and U6 RNAs, U5 RNA is associated with hnRNP particles and is thus probably involved in some, as yet undefined, aspects of pre-messenger RNA processing. In this study, the complete nucleotide sequence of U5 RNA of a dinoflagellate, Crypthecodinium cohnii was determined. The analysis of this dinoflagellate U5 RNA sequence showed that a) the sequence homology between human, rat and chicken U5 RNA sequences and dinoflagellate U5 RNA sequence is 64%; b) the extent and the position of post-transcriptional modifications are similar to those found in U5 RNA of higher eukaryotes; c) although the dinoflagellate U5 RNA is shorter in length (108 nucleotides long vs 117 long in human, rat and chicken cells), the RNA fits well into the same secondary structure proposed for U5 RNA of higher eukaryotes (Krol et al. (1981) Nucl. Acids Res. 9, 769); and d) the AUn nucleotide sequence protected by the Sm-antigen and the tight secondary structure found near the 3'-end of other U-RNAs was also found in dinoflagellate U5 RNA. The high order of homology observed between dinoflagellate U5 RNA and U5 RNA of higher eukaryotes indicates that dinoflagellates are more closely related to metazoans than to early eukaryotes.