Spliced leader RNA sequences can substitute for the essential 5' end of U1 RNA during splicing in a mammalian in vitro system.

L. collosoma or C. elegans SL RNA sequences joined to an adenovirus intron and 3' exon are spliced highly efficiently and accurately in HeLa nuclear extract. After inactivation of U1 snRNPs using RNAase H and a deoxyoligonucleotide complementary to the first 12 nucleotides of U1, splicing of SL RNA-containing constructs continues undiminished, whereas control substrates no longer splice. Since neither binding of U1 snRNPs nor inhibition of splicing is detected using anti-(U1)RNP antibodies, splicing of SL RNA-containing constructs may be entirely U1 snRNP independent. Analyses of altered L. collosoma constructs revealed that the sequence surrounding the 5' splice site is not sufficient to confer U1-independent splicing; the smallest U1-independent region identified so far retains only the first stem-loop of the SL RNA. That sequences responsible for recognition of the 5' splice site can be relocated within the splicing substrate itself reinforces the similarity between group II self-splicing and spliceosome-mediated pre-mRNA splicing.