Polysomes

The rapid repression of pre-existing protein synthesis caused by anaerobic treatment is correlated with a near complete dissociation of polysomes in primary roots of soybeans (Lin Key, 1967) and maize (E.S. Dennis and A.J. Pryor, personal communication). This does not result from degradation of aerobic mRNAs, because the mRNAs encoding the pre-existing proteins remain translatable in an in vitro system at least five hours after anaerobic treatment is initiated (Sachs et al., 1980). This is in agreement... 

Dhindsa, R.S. Bewley, J.D. (1976). Plant desiccation polysome loss not due to ribonuclease. Science, 191,181-2. 

What could be the signal for the induction of the cold shock proteins It has been observed that shifting E. coli cells from 37 to 5 °C results in an accumulation of 70S monosomes with a concomitant decrease in the number of polysomes [129]. Further, it has been shown that a cold shock response is induced when ribosomal function is inhibited, e.g. by cold-sensitive ribosomal mutations [121] or by certain antibiotics such as chloramphenicol [94]. These data indicate that the physiological signal for the induction of the cold shock response is inhibition of translation caused by the abrupt shift to lower temperature. Then, the cold shock proteins RbfA, CsdA and IF2 associate with the 70S ribosomes to convert the cold-sensitive nontranslatable ribosomes into cold-resistant translatable ribosomes. This in turn results in an increase in cellular protein synthesis and growth of the cells. 

A ribosome is a cytoplasmic nucleoprotein stmcture that acts as the machinery for the synthesis of proteins from the mRNA templates. On the ribosomes, the mRNA and tRNA molecules interact to translate into a specific protein molecule information transcribed from the gene. In active protein synthesis, many ribosomes are associated with an mRNA molecule in an assembly called the polysome. 

Many ribosomes can translate the same mRNA molecule simultaneously. Because of their relatively large size, the ribosome particles cannot attach to an mRNA any closer than 35 nucleotides apart. Multiple ribosomes on the same mRNA molecule form a polyribosome, or polysome. In an unrestricted system, the number of ribosomes attached to an mRNA (and thus the size of polyribosomes) correlates positively with the length of the mRNA molecule. The mass of the mRNA molecule is, of course, quite small compared with the mass of even a single ribosome. 

Measuring the Distribution of miRNP Components in Polysome Profiles 99... 

Genome-Wide Analysis of mRNA Polysomal Profiles with... 

Walter, P., and Blobel, G. (1981b). Translocation of proteins across the endoplasmic reticulum. II. Signal recognition protein (SRP) mediates the selective binding to microsomal membranes of in cdro-assembled polysomes synthesizing secretory protein. J. Cell Biol. 91, 551-556. 

Yang, F., Peng, Y., Murray, E. L., Otsuka, Y., Kedersha, N., and Schoenberg, D. R. (2006). Polysome-bound endonuclease PMR1 is targeted to stress granules via stress-specific binding to TIA-1. Mol. Cell Biol. 26, 8803—8813. 

The following procedure works well for resolving polysomal complexes in... 

Example polysome profiles from sucrose gradient fractionation of HeLa cell lysates, either untreated or treated with EDTA, are shown in Fig. 6.4A and B. The polysome profile of untreated HeLa lysates shows three defined peaks in less dense fractions (6 to 9), which correspond to the 80S, 60S, and 40S peaks (Fig. 6.4A). Treatment of lysates with 30 /iM EDTA results in ribosome dissociation leaving predominantly free 60S and 40S subunits... 

Comment A simple test that is often done is to treat the lysate with 30 fiM EDTA prior to loading on the gradient, which leads to the disassembly of polysomes due to chelation of Mg2+ (Fig. 6.4B). EDTA is, however, not a very specific reagent and instead more specific drugs such as puromycin, which causes polypeptide chain termination and disaggregates polysomes, may be used. 

Bagni, C., Mannucci, L., Dotti, C. G., and Amaldi, F. (2000). Chemical stimulation of synaptosomes modulates alpha —Ca2+/calmodulin-dependent protein kinase II mRNA association to polysomes. J. Neurosci. 20, RC76. 

Zalfa, F., Achsel, T., and Bagni, C. (2006). mRNPs, polysomes, or granules FMRP in neuronal protein synthesis. Curr. Opin. Neurobiol. 16, 265-269. 

---