Detailed analysis of the ribosomal RNA synthesis in yeast

In order to study the biosynthesis of ribosomal RNA in Saccharomyces carlsbergensis the labelling kinetics of the various precursor and mature rRNA species were determined using pulse-labelling of protoplasts with [5-3H] uridine at 15 degrees C. Label appears almost immediately in 37 S RNA, the precursor common to both 26 S and 17 S rRNA. Labelled 29 S and 18 S RNA, the immediate precursors of 26 S and 17 S rRNA respectively, were found to appear about 4 min and about 8 min after addition of the isotope respectively. These data indicate that the topography of the 37 S precursor RNA is: 5'-17 S -26 S-3'. The pool size of 29 S RNA is about twice as large as that of either 37 S or 18 S RNA, indicating that under the conditions used processing of 18 S to 17 S rRNA proceeds more rapidly than processing of 29 S to 26 S rRNA. The labelling kinetics of 5.8 S rRNA are in agreement with the existence of a 7 S precursor rRNA, the identity of which was previously established (Trapman, J., de Jonge, P. and Planta, R.J. (1975) FEBS Lett. 57, 26--30) and which, in turn, probably is derived from 29 S precursor rRNA. The labelling kinetics of 5 S rRNA suggest that 5 S RNA sequences, rather than also being part of the common 37 S precursor, are located on a separate primary transcription product. Whether this transcript still contains excess sequences remains to be determined. However, because of the rapid appearance of labelled 5 S RNA, such a precursor would have to be very short lived.