On the Transcriptional Organization of the Ribosomal RNA Genes of Yeast

Abstract

It has been well established by our previous studies that the 26-S and 17-S ribosomal RNA (rRNA) genes in yeast form part of a large common transcriptional unit: the rRNA synthesis in yeast starts with the formation of a 42-S precursor molecule, which contains the sequences of 26-S and 17-S rRNA as well as nonribosomal segments, the latter comprising about 40% of the 42-S precursor sequence (Retèl and Planta, 1970; Planta et al., 1972; van den Bos and Planta, 1973). This excess RNA is removed in a number of discrete steps during the subsequent maturation process. Approximately 140 42-S transcriptional units are present in yeast per haploid genome (Retèl and Planta, 1968; Schweizer et al., 1969). They are grouped together in a rather limited number of clusters (Retèl and Planta, 1972; Cramer et al., 1972; Goldberg et al., 1972). It is possible to isolate by Hg²⁺-Cs₂SO₄ gradient centrifugation a high-molecular-weight DNA fraction, representing 10–12% of the total nuclear DNA, which contains all transcriptional units for rRNA (Fig. 1). About 60% of this so-called ribosomal DNA (rDNA) consists of sequences coding for 42-S precursor rRNA (Retèl and Planta, 1972; Retèl and van Keulen, 1975). The precursor cistrons are interspersed with (A+T)-rich sequences, which make up most of the remaining 40% of the rDNA.