IN-VIVO EXPRESSION OF A VARIANT HUMAN U6 RNA FROM A UNIQUE, INTERNAL PROMOTER

We previously isolated a variant of the human U6 small nuclear RNA gen e (87U6) and demonstrated that transcription of this gene is controlle d by a novel internal promoter. It has now been shown that two blocks of sequence within the coding region are both necessary and sufficient to direct expression of 87U6 in transcription assays performed in vit ro. In addition, 87U6 is expressed in vivo and can assemble into snRNP complexes. Specific primer extension assays on total RNA from HeLa ce lls shows that 87U6 RNA is present in these cells. Also, microinjectio n of plasmid encoded 87U6 genes into Xenopus laevis oocyte nuclei resu lts in the expression of this variant RNA. Immunoprecipitation with an ti-Sm antibodies suggests that 87U6 RNA assembles into a snRNP particl e with U4 snRNA. Finally, the variant snRNA is capped with the U6 spec ific gamma-monomethyl phosphate cap when incubated in HeLa extracts. T hese data suggest that 87U6 RNA may function in the splicing process, in a manner similar to the wild-type U6 RNA. The recent observations o f a minor class of mRNA introns that are spliced by a distinct collect ion of snRNP particles suggest an important role for variant snRNAs in the splicing of transcripts with alternative splice junctions.