A new type of ribonucleoprotein constituent of the polytene nucleus of the salivary glands ofChironomus thummi andCh. tentans

Using electron spectroscopic imaging, a new type of small granular structural constituent has been observed in the extrachromosomal zone of the polytene nucleus of the salivary gland cells ofChironomus thummi andChironomus tentans. These granules appear isolated or in small clumps and are often seen to be connected with surrounding thin fibrils. They are stained by the EDTA procedure, which is preferential for nuclear ribonucleoprotein (RNP) constituents, and by the bismuth oxynitrate method for visualizing phosphorylated compounds. The granules are 15–23 nm in diameter and are digested by prolonged post-embedding RNAse hydrolysis. These structural elements contain the highest concentration of phosphorus in the interchromosomal space as revealed by electron energy loss spectroscopy. The small granules exhibit several morphological and cytochemical features in common with interchromatin granules, but they are not labeled with antibodies directed against extranucleolar small nuclear RNPs (snRNPs), as are interchromatin granules.