The Balbiani ring 6 induction in Chironomus

Balbiani ring (BR) genes in polytene chromosomes of Chironomus salivary glands code for secretory proteins of the sp-I family, 10(6) D. They are used by the aquatic larva to spin a housing and feeding tube. The expression of the BR gene family undergoes correlated changes depending on the environment. In the presence of certain sugars, ethanol or glycerol the normally most active BR2 regresses and its products disappear. There is a parallel induction of a new BR, BR6, and a new sp-I protein. The change seems to represent an adaptative response to phosphate depletion in the larval haemolymph produced by the inducing agents. The BR2 (and BR1) products are heavily phosphorylated and the BR6 product non-phosphorylated. One of the BR2-coded sp-I proteins is cleaved off close to the C-terminus and material with properties expected of the resulting polypeptide can be recovered in the nuclei, accumulating in the BR. This might represent a feed-back signal from translation to transcription.