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

Abstract

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.

Assembling of AcrB Trimer in Cell Membrane

“…Membrane-bound polysomes have been observed under EM in several studies, in which polysomes adopt regular structures. 34–37 For example, Christensen and Bourne found that, in cultured fibroblasts and thyroid epithelial cells, large membrane-bound polysomes exist in hairpin or spiral arrangements. 37 In E. coli , translation of proteins for export, including periplasmic and outer-membrane proteins, had been demonstrated to occur on membrane-bound polysomes.…”

Constant and variable parts in the Balbiani ring 2 repeat unit and the translation termination region

“…Such sequences may occur in connection with translation initiation (Lomedico et al, 1979;Tsujimoto and Suzuki, 1979) and also in the 3'-flanking regions, but are not obviously correlated to translation termination (Busslinger et al, 1979;Honjo et al, 1979). The distribution of ribosomes on putative translation complexes with BR2 mRNAs makes it unlikely that the translation and transcription termination sites are close together (Francke et al, 1982). The palindromes around the translation termination site could suggest a local control region.…”

“…These peptides are extremely large and have an unusual amino acid composition (Grossbach, 1969;Hertner et al, 1980). The giant size is reflected in the translation unit of the protein Francke et al, 1982).…”

Nuclear localization of a DNA-binding C-terminal domain from Balbiani ring coded secretory protein.

“…Ribosomes are seen to lose their nascent sp-I molecules briefly before the 3'-end of the mRNA while continuing themselves without visible polypeptide chains for a short distance on the mRNA (Francke et al, 1982). Basic dipeptide sites are of interest as possible proteolytic processing sites.…”