The Balbiani ring particle: a model for the assembly and export of RNPs from the nucleus?

Balbiani rings are exceptionally large puffs on the polytene chromosomes in the dipteran Chironomus tentans. These puffs are particularly well suited for studies of the structure of active genes and the synthesis and transport of specific RNA-protein (RNP) particles. The Balbiani ring RNP particle consists of a ribbon bent into a ring-like structure, the ribbon being built from a tightly folded 7 nm RNP filament. The assembly of the particle takes place concomitant with transcription and occurs in a stepwise fashion. As the particle is transported through the nuclear pore the ribbon straightens out, with the 5' end of the transcript leading. During translocation the tightly packed RNP filament is gradually unfolded, and on the cytoplasmic side the mRNA is immediately engaged in polysome formation.