RNA to protein ratio

A third type of mRNA-protein complex has been derived from cytoplasmic polysomes (Perry and Kelley, 1968 Henshaw, 1968). It was shown that after dissociation of polysomes into subunits by EDTA treatment, mRNA is liberated in the form of ribonucleoprotein with an RNA to protein ratio equal to 1 2 or 1 3. 

In both trout and crabs the short-term increases in fractional rates of protein synthesis that occur with feeding after a fast of several days are brought about with no significant change in the RNA concentrations in the tissues (Fig. 19a,b). An increase in the activity of the RNA is therefore the predominant mechanism responsible for the stimulation in protein synthesis (Table 3). The slopes of the lines relating fractional protein synthesis rates and RNA to protein ratios increase in both animals (Fig. 19). Increases in Icrna have been found in mammals (Garlick et al. 

These increases in RNA to protein ratios with increased feeding and growth rates and decreases with starvation support the idea that RNA concentrations indicate the capacity of the tissues to synthesise proteins and hence grow. The rates of change of RNA concentration with a new nutritional level are probably determined by such factors as the size of the animal and temperature (e.g. Miglavs and Jobling 1989). 

At the moment the RNA to protein ratio, which particularly in white muscle seems to correlate with long term nutritional status, growth and protein synthesis rates, would seem to be a more suitable ratio for expressing RNA concentration in the tissues than RNA to DNA ratios. RNA to protein ratio measurements on freshly caught wild animals could provide an indicator of the growth rate at the time of capture as has been suggested for fish (Buckley 1984). 

Archaea exhibit intralineage variations of their ribosome type. To generalize, all archaea contain ribosomes (especially their small subunits) that are considerably richer in protein than those of bacteria, with the exception of the Halobacteriales and of methanogens belonging to the Methanomicrobiales and Methanobacteriales. These differ from other archaea in harbouring ribosomes having the same protein mass and the same (3 2) RNA to protein mass ratio as those of typical bacteria. Because the size and the secondary structure of the archaeal rRNAs do not differ from those of bacterial rRNAs, it is likely that the accreted proteins do not interact directly with the rRNA moiety. 

Another approach is to measure the buoyant density of the particles using CsCl density gradient ultracentrifugation. If the strands of RNA between the SOS particles in the polysomelike complexes are short (as may be expected from electron micrographic evidence) the ratio of RNA to protein and consequently the buoyant density of the heavy particles and of SOS particles should be the same. If the RNA strands between particles are long, this ratio and the density should be higher. The experiments performed with... 

It is worth mentioning in this context that there seems to be a general difference between RNA and protein landscapes Certain amino acid composition ratios between hydrophobic and hydrophilic amino acids presumably give rise to insoluble aggregates and this may lead to holes in protein sequence space. Perhaps, the concept of holey adaptive landscapes as favored in a series of recent papers on models of evolution [62] might be useful in this context. 

A simple measure of DNA purity is to determine the ratio of OD26o/OD28o. A pure DNA sample will have a ratio of 1.8-2.0 (80,81). A ratio lower than 1.8 raises concerns about high levels of contamination with RNA or protein. A spec-trophotometric scan (absorbance 230-340 nm) is also useful to evaluate protein contamination since protein scatters light much more effectively at 230 nm and the presence of protein may shift the absorbance of DNA to slightly greater than 260 nm (170). 

Once the mRNP reaches the cytoplasm, most of the mRNP proteins that associated with the mRNA in the nucleus, the nuclear cap-binding complex, and the nuclear poly (A)-binding protein (PABPII) dissociate and are shuttled back to the nucleus. In the cytoplasm, the 5 cap of an exported mRNA is bound by the eIF4E translation initiation factor, the poly(A) tail is bound by multiple copies of the cytoplasmic poly(A)-binding protein (PABPI), and other RNA-binding proteins associate with the body of the mRNA, forming a cytoplasmic mRNP that has a lower ratio of protein to RNA than nuclear mRNPs. 

The spectroscopic properties of a DNA solution can be used to determine the approximate purity of the preparation as well. Because the shape and °f the UV absorption spectra will vary depending on the environment of the bases, one can use the ratio of A260nrr/A280nm estimate the RNA and protein content of a dsDNA preparation. Pure dsDNA has a ratio of 1.8, and pure RNA has a ratio of 2.0. Because protein has a °f around 280 nm, the A260nrr/... 

Although the DNA/protein ratio seems to be fairly constant in bacteria, the RNA content increases with the growth rate (Herbert, 1961 Dicks and Tempest, 1966) (Fig. 10). Therefore, it will be possible to decrease the RNA content by growing the cells at a low growth rate after they have been grown exponentially at the maximum rate. 

In 1964 Spirin et al. discovered in embryonic cells cytoplasmic ribonucleoprotein particles containing rapidly labeled RNA with many properties of mRNA. The particles contain RNA and protein in a ratio of about 1 3 or 1 4. These particles, which are free cytoplasmic mRNA-protein complexes not combined with ribosomes, were named informosomes. Under ultracentrifugation they form a number of discrete peaks with sedimentation coefficients equal to 20, 30, 40, 50, 55, 65, and 75S (Spirin et al., 1964). 

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