Role for small nuclear RNAs in "programming" chromosomal information?

Abstract

ALTHOUGH small nuclear RNAs (snRNAs) have been studied extensively1,2, almost nothing is known about how they function in cells. I present here the results of a detailed study of the behaviour of snRNAs during mitosis, which has uncovered what may be the first important clue as to their function. I suggest, from observations of the pattern of snRNA association with mitotic chromosomes, that snRNAs are involved in specifying which genes are transcribed. Between 4 and 10 electrophoretically distinct snRNAs are found in species from protozoa to mammals, ranging from 65 to 200 nucleotides long. The snRNAs, which generally comprise less than 0.5% of total cellular RNA, are largely confined to the nuclei during interphase; some, however, are probably present in the interphase cytoplasm–although at concentrations much below that in the nucleus3. Such molecular characteristics as base compositions, methylation patterns and nucleotide sequences have been determined for at least some snRNAs, but have provided no recognisable clues regarding function. No evidence is available that snRNAs are part of the cellular translation machinery, as is true for the more abundant transfer, ribosomal and messenger RNAs, and their metabolic stability argues against a function as primers in DNA replication4. It has been suggested that certain snRNAs are related to ribosomal RNA processing2, cell proliferation5 and gene expression—but no data in support of these views are definitive. The snRNAs have been shown to be released to the cytoplasm during division, then to return virtually intact to the post-division daughter nuclei6,7.