THE U5 RNA OF TRYPANOSOMES DEVIATES FROM THE CANONICAL U5 RNA - THE LEPTOMONAS-COLLOSOMA U5 RNA AND ITS CODING GENE

Fractionation of the abundant small ribonucleoproteins (RNPs) of the t rypanosomatid Leptomonas collosoma revealed the existence of a group o f unidentified small RNPs that were shown to fractionate differently t han the well-characterized trans-spliceosomal RNPs, One of these RNAs, an 80-nt RNA, did not possess a trimethylguanosine (TMG) cap structur e but did possess a 5' phosphate terminus and an invariant consensus U 5 snRNA loop 1, The gene coding for the RNA was cloned, and the coding region showed 55% sequence identity to the recently described U5 homo logue of Trypanosoma brucei [Dungan, J. D., Watkins, K. P. & Agabian, N. (1996) EMBO J. 15, 4016-4029], The L. collosoma U5 homologue exists in multiple forms of RNP complexes, a 10S monoparticle, and two subgr oups of 18S particles that either contain or lack the U4 and U6 small nuclear RNAs, suggesting the existence of a U4/U6.U5 tri-small nuclear RNP complex, In contrast to T. brucei U5 RNA (62 nt), the L. collosom a homologue is longer (80 nt) and possesses a second stem-loop, Like t he trypanosome U3, U6, and 7SL RNA genes, a tRNA gene coding for tRNA( Cys) was found 98 nt upstream to the U5 gene, A potential for base pai r interaction between U5 and SL RNA in the 5' splice site region (posi tions -1 and +1) and downstream from it is proposed, The presence of a US-like RNA in trypanosomes suggests that the most essential small nu clear RNPs are ubiquitous for both cis- and trans-splicing, yet even a mong the trypanosomatids the U5 RNA is highly divergent.