Heat-shock induced puffing changes in Balbiani rings

A 5 minutes exposure of Chironomus larvae to near-lethal temperatures (39–40° C) produces a characteristic sequence of puffing changes in the Balbiani rings of the 4th salivary gland chromosomes: An initial phase of complete puff regression is followed, after a variable time lag, by a phase of rapid recovery to overnormal puff sizes. This is accompanied by RNP droplet formation. RNA synthesis at Balbiani rings during the initial puff regression still occurs. Regression is inhibited by 2,4-dinitrophenol, while recovery can be prevented by both 2.4dinitrophenol and actinomycin D. Regression and recovery are insensitive to cycloheximide. RNP droplets, as observed in Balbiani rings, in the nuclear sap and at the nucleolar rim, are composed of a fine fibrillar matrix which is covered by Balbiani ring granules in various phases of assembly. The results are discussed in terms of a model of puffing based on an equilibrium between RNA synthesis, RNA processing and RNP release from the puff.