Two genes in Balbiani ring 2 with metabolically different 75S transcripts.

Balbiani ring 2 (BR2) in salivary glands of Chironomus pallidivittatus and C. tentans (two sibling species of the subgenus Camptochironomus) is a favoured model system for studies of gene organization and transcript formation. Here we show that BR2 is more complex than hitherto believed, containing two 75S RNA-producing genes, BR2a and BR2b, present in different 35-40 kb blocks of DNA. The transcripts hybridizing to two different repeat units originating in BR2 differ in size. Further support for the presence of two genes comes from RNA studies during experimentally induced BR2 regression. The amounts of BR2a RNA per cell remain more or less constant throughout the course of the experiment, whereas the BR2b RNA decreases considerably. Under normal conditions there is 5-6 times more BR2a RNA than BR2b RNA. This ratio increases 3-fold under experimental conditions. BR2a and BR2b, although partially homologous, contain repeat units with characteristic differences. BR2a contains a repeat unit that is much more similar to a BR1 repeat than it is to the BR2b repeat. The possibility is discussed that a Balbiani ring in general represents an integrated set of active genes rather than a singe gene.