Delayed flow-through cytoplasm of newly synthesized Balbiani ring 75S RNA.

With a nonaqeous microdissection technique, the cytoplasm of Chironomus salivary gland cells can be separated into concentric zones situated at increasing distances from the nuclear envelope. This dissection technique is used here to investigate the cytoplasmic distribution of 75S RNA of Balbiani ring origin. The Balbiani ring 75S RNA has properties of a messenger RNA coding for secretory proteins. After a pulse of RNA precursor to the living animal, labeled Balbiani ring 75S RNA is found mainly in the cytoplasm located closer to the nuclear envelope, with smaller amounts toward the periphery of the cell. This gradient, initially very steep, lasts for a least 2 days, but less than 6 days. Experiments with 5-fluorouridine indicate that the formation of the gradient does not depend upon simultaneous export of ribosomal subunits. After a pretreatment of the animals with the protein synthesis inhibitor cycloheximide, however, newly synthesized 75S RNA distributes evenly in the cytoplasm-that is, this treatment prevents the formation of the 75S RNA gradient. The gradient in salivary glands of normally cultured animals is therefore likely to be the result of diffusion restriction of the labeled 75S RNA. Thus the 75S RNA located closer to the nuclear envelope is the most recently exported 75S RNA. An explanation of these results is the the 75S RNA associates with the membranes of the endoplasmic reticulum early or immediately after nuclear release. This association should occur in the cytoplasm surrounding the nucleus and may occur either as single particles and/or as parts of polysomes.