Precursors of U4 small nuclear RNA.

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Abstract
The processing and ribonucleoprotein assembly of U4 small nuclear RNA was investigated in HeLa cells. After a 45-min pulse label with [3H]uridine, a set of apparently cytoplasmic RNA was observed migrating just behind the gel electrophoretic position of mature U4 RNA. These molecules were estimated to be 1 to at least 7 nucleotides longer than mature U4 RNA. They reacted with Sm autoimmune patient sera and a monoclonal Sm antibody, indicating their association with proteins characteristic of small nuclear ribonucleoprotein complexes. The same set of RNA was identified by hybrid selection of pulse-labeled RNA with cloned U4 DNA, confirming that these are U4 RNA sequences. No larger nuclear precursors of these RNA were detected. Pulse-chase experiments revealed a progressive decrease in the radioactivity of the U4 precursor RNA coincident with an accumulation of labeled mature U4 RNA, confirming a precursor-product relationship.