Unusual processing of nucleolar RNA synthesized during a heat shock in CHO cells

Maturation of pre-rRNA has been investigated through heat shock experiments in which pre-rRNA synthesis is successively turned off and turned on. After one hour at 43 degrees C high molecular weight RNA is no longer synthesized and both the methylation and the maturation of pre-rRNA synthesized before heat shock are blocked. After two hours recovery at 37 degrees C, methylation and simultaneous maturation of pre-existing RNA occur while pre-rRNA synthesis is reinitiated only after 7 hours at 37 degrees C. During the first 30 min. at 43 degrees C, a residual synthesis of high molecular weight RNA is observed in the nucleolus with an average molecular weight slightly higher than pre-rRNA (4.6 10(6)). During the recovery period at 37 degrees C, RNA synthesized at 43 degrees C is slowly processed into unusual species (39S, 35S, 29S). No new ribosomal RNA appeared in the cytoplasm. This unusual maturation pathway could be a minor pathway of nucleolar RNA processing in exponentially growing cells.