Post-transcriptional regulation of the U3 small nucleolar RNA.

A high copy shuttle vector was used to express a "tagged" U3 small nucleolar RNA (snoRNA) gene in Schizosaccharomyces pombe to examine regulatory responses to a high gene dosage. RNA analyses utilizing reverse transcription-PCR amplification and restriction fragment length polymorphism indicated that the tagged gene was both proportionally and highly expressed and that downstream processing and/or termination were critical to U3 snoRNA stability. In contrast, direct measurements of the total cellular U3 snoRNA showed essentially normal levels of mature RNA, although measurements of precursor levels confirmed a highly expressed gene construct. Taken together, the results indicated that the steady state amounts of mature U3 snoRNA were primarily regulated at the post-transcriptional level. This regulatory mechanism prevents over-accumulation of the cellular U3 snoRNA and can efficiently degrade mutant RNA molecules. Together with past studies on other 3' extended RNA precursors, the results support post-transcriptional regulation as a quality control mechanism in which appropriate amounts of functional RNA are stabilized by protein interaction while excess or defective RNA is rapidly degraded. Precursor processing in vitro and mutational analyses were consistent with this model.