Transcriptional Units for Ribosomal Proteins in Yeast

Abstract

The effects of ultraviolet irradiation on the rates of synthesis of individual ribosomal proteins in yeast were examined and compared with the ultraviolet sensitivities of the synthesis of other yeast proteins. It was found that the synthesis of yeast ribosomal proteins is much more sensitive to ultraviolet irradiation than that of other yeast cellular proteins. Taking into account the half-life of yeast mRNA, the results obtained indicate that the genes coding for ribosomal proteins form part of long transcriptional units, which are much longer than the DNA stretch needed to code for a ribosomal protein of average molecular weight.Saturation hybridization of total poly(A)-containing mRNA with yeast nuclear DNA revealed that as much as 30 % of DNA is complementary to yeast mRNA. Thus, the primary transcript of a protein gene on the average is about 1.7 times the length of the actual messenger.On the basis of the various experimental data we suggest a clustering of the yeast ribosomal protein genes in a number of common transcriptional units.Knowledge about the genetic organization of all ribosomal constituents is necessary to get insight into the way in which the synthesis of these molecules is coordinated. Both in prokaryotic (see [l] for a review) and in eukaryotic [2,3] organisms the individual ribosomal RNA components are transcribed from a common transcriptional unit. An attractive possibility would be that the ribosomal protein genes are also arranged in one or more polycistronic units. From genetic studies it appeared that the ribosomal protein genes in Eschevichia coli are clustered (see [4] for a review). Experiments with rifampicin also indicated that the bacterial ribosomal protein genes are organized in a limited number of common transcriptional units [5]. Very recently, this could be confirmed by measuring the ultraviolet sensitivities of the rates of synthesis of the ribosomal proteins [6]. At least six transcriptional units for ribosomal proteins were found to exist, of which the longest carries 10-12 ribosomal protein genes of average size [6].So far, nothing is known about the genes specifying the ribosomal proteins in eukaryotic cells. In an attempt to determine the size of the transcriptional units for ribosomal proteins in yeast, we measured the sensitivity of the synthesis of individual ribosomal proteins to ultraviolet light by irradiation of whole cells. This technique has been succesfully employed not only for prokaryotic systems but also in the study of the organization of the ribosomal RNA genes in eukaryotes (both in yeast [7] and in mammalian cells [S]). The method is based on the finding that lesions in the DNA, induced by ultraviolet light, cause premature termination of the transcription, resulting in the release of the incomplete transcript and the RNA polymerase from the template (see [7] for more details). Therefore, the DNA located between the damaged site and the next promotor site will not be transcribed. Since genes represent stretches of DNA large enough to allow the assum...