The molecular weight of rRNA precursor molecules and their processing in higher plant cells.

Actively dividing callus cells of higher plants (Petroselinum crispum, Daucus carota, Acer pseudoplatanus) were used to detect the primary gene product of rDNA in vivo. Parsley and carrot cells were labelled with [32P]orthophosphate. Under non-denaturing conditions, in both cases only one high molecular weight rRNA precursor was present on polyacrylamide gels. Its molecular weight did not exceed 2.5 x 10(6) dalton. Under denaturing conditions, 2.0--2.1 x 10(6) dalton were determined on formamide gels. This rRNA precursor was already present after a labelling period of 5--10 min. In parsley cells labelled mature rRNA (25S and 18S) arrived in the cytoplasm 45 min after onset of incubation. In Acer pseudoplatanus incubated with [3H]uridine two rapidly labelled components did emerge from polyacrylamide gels without formamide; their molecular weights were 2.3 and 3.2--3.4 x 10(6) dalton. After electrophoresis in formamide, the larger component disappeared, thus indicating that it would be an intermolecular aggregate of different RNAs. From these results we have no evidence for the existence of rRNA precursors exceeding the molecular weight of 2.5 x 10(6) dalton.