Sequences required for 3' end formation of human U2 small nuclear RNA.

Xenopus oocytes injected with human U2 snRNA genes synthesize mature U2 as well as a U2 precursor with about 10 extra 3' nucleotides (human pre-U2 RNA). Formation of the pre-U2 3' end requires a downstream element located between position +16 and +37 in the U2 3'-flanking sequence. The distance between this element and the U2 coding region can be increased without affecting formation of the pre-U2 3' end. When the natural sequence surrounding the pre-U2 3' end is changed, novel 3' ends are still generated within a narrow range upstream from the element. The 3' terminal stem-loop of U2 snRNA is not required for pre-U2 3' end formation. A sequence within the 3' element (GTTTN0-3AAAPuNNAGA) is conserved among snRNA genes transcribed by RNA polymerase II. Our results suggest that the 3' ends of pre-U2 RNA and histone mRNA may be generated by related but distinct RNA processing mechanisms.