THE TURNOVER AND SUBCELLULAR LOCALIZATION OF THE CHICK FOREBRAIN SMALL NUCLEAR RNAs

Analysis of 2 day old chick forebrain RNA on polyacrylamide gels revealed the presence of the 29S, 18S, 5.5s and 5s rRNAs, 4S tRNA and the small mol wt nuclear RNA (snRNA) species K, L, C, DD and G. The turnover of these molecules was studied following the intracerebral injection of ³H]uridine into 2 day old animals. The specific activities of these molecules declined over a 23 day time course and all displayed first-order decay kinetics. Apparent half-lives (in days) for 29S (7). 18S (S), L (24), C (6.5), DD′ (8.5), 5.5S (7), 5S (13), G (6) and 4S (7) were obtained. The subcellular localization of the snRNAs was studied in 5 day old chick forebrain. The snRNAs accounted for almost 17% of nuclear RNA, in contrast to their low levels in whole forebrain (1.2%). This was due to an approx 20-fold nuclear enrichment of C, DD′, G′ and H. Relatively low levels of K and L were present within the nuclear fraction. However, K and L, and to a lesser extent C and DD, were readily detected within the post-microsomal supernatant. Production of a pH 5 enzyme fraction from this supernatant resulted in a 2-3-fold enrichment of K, L, C and DD′ with respect to tRNA. The polysome containing fractions also displayed significant levels of L with about 1 mol of L per 30 of 5S rRNA. A new method for the determination of the subcellular distribution of the snRNAs is described. It depends upon the purity of the nuclear, microsomal and cell sap subfractions rather than upon total recoveries from such fractions. Utilizing the relative amounts of these RNA species within whole forebrain and the major subfractions, distributions (as a percentage within each fraction) were obtained. The observation of significant levels of K, L, C and DD within the cytoplasmic fractions raises some doubts about previous suggestions of an exclusively nuclear role for these molecules. It is proposed that the snRNAs may be involved in a multistage interaction within the process of eukaryotic gene expression.