Electron microscopic visualization of a discrete class of giant translation units in salivary gland cells of Chironomus tentans

Lysates were made of whole salivary glands from Chironomus tentans and electron microscopic spreads prepared according to Miller and Beatty (1969). The sedimented material consisted mainly of ribosomes, most of which were present in giant polysomes which showed gradients of material protruding laterally from the ribosomes, interpreted as nascent polypeptide chains. Each giant polysome contained one such gradient. On the basis of the presence of a small group of terminal ribosomes without nascent chains at the 3′ end, 19 apparently complete polysomes were selected, representing a discrete class of polysomes containing between 66 and 92 ribosomes (79 ± 7, m ± s.d.). Arguments are presented that they constitute the translation units for giant secretory proteins coded by RNA from the large Balbiani rings, BR1 and BR2.