Evidence for the presence of a small U5‐like RNA in active trans‐spliceosomes of Trypanosoma brucei.

The existence of the Trypanosoma brucei 5′ splice site on a small RNA of uniform sequence (the spliced leader or SL RNA) has allowed us to characterize the RNAs with which it interacts in vivo by psoralen crosslinking treatment. Analysis of the most abundant crosslinks formed by the SL RNA allowed us previously to identify the spliced leader‐associated (SLA) RNA. The role of this RNA in trans‐splicing, as well as the possible existence of an analogous RNA interaction in cis‐splicing, is unknown. We show here that the 5′ splice site region of the SL RNA is also crosslinked in vivo to a second small RNA. Although it is very small and lacks a 5′ trimethylguanosine (TMG) cap, the SLA2RNA possesses counterparts of the conserved U5 snRNA stem‐loop 1 and internal loop 1 sequence elements, as well as a potential trypanosome snRNA core protein binding site; these combined features meet the phylogenetic definition of U5 snRNA. Like U5, the SLA2 RNA forms an RNP complex with the U4 and U6 RNAs, and interacts with the 5′ splice site region via its putative loop 1 sequence. In a final analogy with U5, the SLA2 RNA is found crosslinked to a molecule identical to the free 5′ exon splicing intermediate. These data present a compelling case for the SLA2 RNA not only as an active trans‐spliceosomal component, but also for its identification as the trypanosome U5 structural homolog. The presence of a U5‐like RNA in this ancient eukaryote establishes the universality of the spliceosomal RNA core components.