Evidence for transport of preribosomal RNA from the nucleolus to the chromosomes in Chironomus tentans salivary gland cells

Chromosomal RNA from the salivary gland cells of Chironomus tentans has been analysed by gel electrophoresis on agarose-acrylamide composite gels. After short times of labelling (45 min), only heterodisperse high molecular weight RNA was observed. After increased incorporation times (90 to 180 min) first a 23 s and later also a 30 s monodisperse RNA fraction appeared in the chromosomes. The electrophoretic mobility and the kinetics of labelling indicate a nucleolar origin of these fractions, known to be formed from a 38 s RNA precursor in the nucleolus and likely to be precursors to ribosomal RNA.