Localisation of rapidly and slowly labelled nuclear RNA as visualized by high resolution autoradiography

Abstract

Autoradiography combined with electron microscopy is used to visualize the incorporation of 3H-uridine into the nuclei of cultured BSC 1 monkey kidney cells. A preferential RNP stain is applied to differentiate between chromatin and nuclear ribonucleoproteins. It is demonstrated that the incorporation of the radioactive precursor can be localized after pulses of only 2 min., on the one hand at the limit of the chromatin and of fibrillar areas of the nucleolus, and on the other hand in the proximity of condensed chromatin throughout the nucleoplasm where perichromatin fibrils are present. Pulses of 5 to 15 min show a considerable increase of labelling. The number of silver grains over the fibrillar areas of the nucleolus increases. The pattern of extranucleolar incorporation is not changed. If these pulses are followed by a chase varying from 15 min to 3 h, some radioactivity can always be demonstrated throughout the nucleolus, with a considerable amount in the RNP containing interchromatin area as well. However, interchromatin granules are not, or only weakly, labelled even after a labelling of 1 h followed by a chase of up to 3 h. The results are discussed in view of recent biochemical findings of a rapidly labelled and locally metabolized nuclear RNA. It is suggested that the early-labelled RNA visualized in the vicinity of the condensed chromatin may at least partially correspond to the DNA-like RNA or HnRNA fraction.