No abstract

SUMMARYHigh resolution autoradiography of isolated yeast nuclei has been used to investigate the distribution of newly formed RNA in the nucleus. The nuclei were isolated after incubation of spheromasts with 3H-uracil for 5 min. Most grains (80 %) are located over the dense crescent. In the dense crescent the grains seem to be associatedwith.the nucleolonema. An analysis of the nuclear RNA on polyacrylamide gels shows that the incorporated 3H-uracil is predominantly present in the initial (37s) ribosomal precursor RNA. The EM autoradiographic and biochemical data, taken together, strongly suggest that we have to ascribe a nucleolar function to the dense crescent of the yeast nucleus.When isolated yeast nuclei are observed by means of the phase-contrast microscope, a crescent-shaped segment of high optical density can be discerned [l]. EM studies have ascertained it to be very likely that this crescent is equivalent to the mammalian nucleolus. A large part of the "dense crescent" is occupied by a structure strongly resembling the nucleolonema of the mammalian nucleolus. Moreover, a correlated biochemical and electron microscopic study has shown that the dense crescent consists mainly of RNA and protein [2], The exponentially growing yeast cells from which the nuclei are isolated are endowed with a very high rate of synthesis of ribosomal RNA and a high ribosomes content [I]. Thus it is not surprising that yeast nuclei, compared with nuclei of animal cells, contain a relatively large amount of RNA which is mainly ribosomal precursor RNA [3]. EviExptl Cell Res 70 dence has come from several sources [3-51, that in yeast both species of ribosomal RNA derive from a common large precursor molecule. Hence it is clear that yeast, one of the most primitive eukaryotes, possesses a mechanism for the processing of ribosomal RNA which resembles that of higher eukaryotes. This raises the question whether, in yeast, as in higher eukaryotes, the ribosomal precursors will be confined to the region of the nucleus resembling a nucleolus.In order to obtain an insight into the intranuclear localization of the ribosomal precursor RNA, we have performed pulse labelling experiments with 3H-uracil followed by electron microscopic autoradiography. MATERIALS AND METHODS Organism and isotopeThe experiments were performed with spheroplasts [l] of the yeast strain Saccharomyces carlsbergensis,

In vivo methylated precursors of ribosomal RNA in yeast have been characterized on acrylamide gels. The initial ribosomal precursor in the yeast nucleus is a 37s RNA component, which is processed to a nuclear 28s RNA. Both the 37s and the 28s RNA components are important constituents of the yeast nucleus. A possible 33s RNA intermediate has been observed. Newly formed 18s rRNA rapidly enters the cytoplasm, while newly formed 26s rRNA appears later. The 26s rRNA is most probably formed from the nuclear 28s RNA.

No abstract

No abstract