Packing and delivery of a genetic message

Balbiani rings (BRs) 1 and 2 are two exceptionally large chromosomal puffs on chromosome IV in the salivary glands of the dipteran Chironomus tentans. The BR genes are 35-40 kb, contain four short introns, and encode salivary polypeptides of one million molecular weight. They have proven uniquely su ited for visualization of the assembly and transport of a specific messenge r ribonucleoprotein (RNP) particle. A BR transcript is packed with proteins into a thin RNP fibril, which is folded into a compact ring-like structure . The completed BR particle is released from the gene and moves randomly in the nucleoplasm before it becomes associated with the nuclear pore complex . The passage through the nuclear pore is a highly ordered process with a s eries of consecutive steps: initial binding, docking, unfolding, movement t hrough the pore with the 5' end of the transcript in the lead, and exit int o the cytoplasm. On the cytoplasmic side, the RNA becomes immediately engag ed in protein synthesis. Recently, several major proteins in the BR particl e have been identified and characterized. They are added to the BR RNA mole cule concomitantly with transcription. During the ensuing RNA transport, th e various proteins behave differently, some remaining in the nucleus, other s entering the cytoplasm coupled to the RNA. The flow pattern of a given pr otein seems to be closely related to the specific function of the protein. The RNA-binding proteins are likely to play various active roles during gen e expression rather than being solely packaging proteins. Finally, it is em phasized that the co-transcriptional loading of the transcript with protein s is probably a key process in gene expression that to a large extent deter mines the fate of an mRNA both in the nucleus and the cytoplasm.