Assembly and transport of a premessenger RNP particle

Salivary gland cells in the larvae of the dipteran Chironomus tentans offer unique possibilities to visualize the assembly and nucleocytoplasmic trans port of a specific transcription product. Each nucleus harbors four giant p olytene chromosomes, whose transcription sites are expanded, or puffed. On chromosome IV, there are two puffs of exceptional size, Balbiani ring (BR) 1 and BR 2, A BR gene is 35-40 kb. contains four short introns, and encodes a 1-MDa salivary polypeptide. The BR transcript is packed with proteins in to a ribonucleoprotein (RNP) fibril that is folded into a compact ring-like structure. The completed RNP particle is released into the nucleoplasm and transported to the nuclear pore, where the RNP fibril is gradually unfolde d and passes through the pore. On the cytoplasmic side, the exiting extende d RNP fibril becomes engaged in protein synthesis and the ensuing polysome is anchored to the endoplasmic: reticulum. Several of the BR particle prote ins have been characterized, and their fate during the assembly and transpo rt of the BR particle has been elucidated. The proteins studied are all add ed cotranscriptionally to the pre-mRNA molecule. The various proteins behav e differently during RNA transport, and the flow pattern of each protein is related to the particular function of the protein. Because the cotranscrip tional assembly of the pre-mRNP particle involves proteins functioning in t he nucleus as well as proteins functioning in the cytoplasm. it is conclude d that the fate of the mRNA molecule is determined to a considerable extent already at the gene level.