Transient nucleolar localization of U6 small nuclear RNA in Xenopus laevisoocytes

Recent studies on the 2'-O-methylation and pseudouridylation of U6 small nu clear RNA (snRNA) hypothesize that these posttranscriptional modifications might occur in the nucleolus. Ln this report, we present direct evidence fo r the nucleolar localization of U6 snRNA and analyze the kinetics of U6 nuc leolar localization after injection of in vitro transcribed fluorescein-lab eled transcripts into Xenopus laevis oocytes. In contrast to U3 small nucle olar RNA (snoRNA) which developed strong nucleolar labeling over 4 h and ma intained strong nucleolar signals through 24 h, U6 snRNA localized to nucle oli immediately after injection, but nucleolar staining decreased after 4 h . By 24 h after injection of U6 snRNA, only weak nucleolar signals were obs erved. Unlike the time-dependent profile of strong nucleolar localization o f U6 snRNA or U3 snoRNA, injection of fluorescein-labeled U2 snRNA gave wea k nucleolar staining at all times throughout a 24-h period; U2 snRNA modifi cations are believed to occur outside of the nucleolus. The notion that the decrease of U6 signals over time was due to its trafficking out of nucleol i and not to transcript degradation was supported by the demonstration of U 6 snRNA stability over time. Therefore, in contrast to snoRNAs like U3, U6 snRNA transiently passes through nucleoli.