Cytoplasm protein synthesis

Romero-Romero, T., Anaya, A.L. and Cruz-Ortega, R. (2002). Screening for effects of phytochemical variability on cytoplasmic protein synthesis pattern of crop plants. Journal of Chemical Ecology 28 617-629. 

Certain antibiotics (for example, chloramphenicol) inhibit mitochondrial protein synthesis, but not cytoplasmic protein synthesis, because mitochondrial ribosomes are similar to prokaryotic ribosomes. 

Although a few subunits of mitochondrial membrane proteins are coded by mitochondrial DNA and synthesized in the mitochondrial matrix, most membrane proteins including the adenine nucleotide carrier are coded by nuclear genes and synthesized on cytoplasmic ribosomes [80,81], Chloramphenicol, an inhibitor of mitochondrial protein synthesis, does not inhibit incorporation of radioactive leucine into the carrier in growing Neurospora crassa, but cycloheximide, an inhibitor of cytoplasmic protein synthesis, does inhibit leucine incorporation [78]. Also, a yeast nuclear respiratory mutant has been shown to cause a defect in adenine nucleotide transport [81], and the nuclear gene responsible for coding the carrier in yeast is currently being cloned for further studies [82]. 

Since protein synthesis in eukaryotes occurs both in the cytoplasm and in certain cellular organelles such as mitochondria (and chloroplasts in the case of plants), the mechanisms of cytoplasmic protein synthesis are described first, followed by a discussion of the similarities and differences in protein synthesis in the organelles, particularly the mitochondria. 

In addition to being an indispensable amino acid that is present in proteins of animals and humans and that becomes incorporated into proteins during protein synthesis, tryptophan itself has been found to have a regulatory effect on protein synthesis. Tryptophan can stimulate hepatic protein synthesis. Although the mechanism for this regulatory action appears to be complex, it is apparent that this action involves a specific nuclear envelope receptor to which it binds, followed by enhanced nucleocytoplasmic translocation of mRNA, and subsequent increased cytoplasmic protein synthesis. 

The increased rate of ABA biosynthesis in dehydrated leaves can be blocked by inhibitors of transcription, such as actinomycin D and cordycepin [163-165], as well as by cycloheximide, an inhibitor of cytoplasmic protein synthesis [163,165-167]. These results indicate that nuclear gene transcription and cytosolic protein synthesis are required before an increase in ABA biosynthesis can take place. These processes probably account for the lag period prior to ABA accumulation following the onset of stress [94,163]. Water stress and cycloheximide had no effect on the conversion of XAN to ABA [54]. This indicates that the enzymes catalyzing these conversions are constitutively expressed. The most likely step stimulated by dehydration is, therefore, at the level of xanthophyll cleavage, although isomerization of xanthophylls cannot be ruled out. 

DNA is found primarily in the nucleus of eukaryotic cells. Protein synthesis takes place primarily in that part of the cell called the cytoplasm. Protein synthesis requires that two major processes take place the first occurs in the cell nucleus, the second in the cytoplasm. The first is transcription, a process in which the genetic message is transcribed onto a form of RNA called messenger RNA (mRNA). The second process involves two other forms of RNA, called ribosomal RNA (rRNA) and transfer RNA (tRNA). 

In fact it has been shown that during the nutritional shift-down accompanying sporulation in yeasts, regulation of total RNA synthesis rests on the interlock between the mitochondrial and cytoplasmic protein synthesis. 

Positive regulator of the mitochondrial or cytoplasmic protein synthesis controlling specifically the synthesis of at least one component of each deficient activity (cytochrome oxidase, ATPase, and X), but allowing the synthesis of other components, such as cytochrome b. 

These facts bring us to one of the major outstanding problems in mitochondrial biogenesis what is the mechanism whereby the vast majority of mitochondrial proteins, coded for by nuclear DNA and synthesized on cytoplasmic ribosomes, traverse the mitochondrial membrane barriers and enter the closed organelle Since the mitochondrion is impermeable to proteins of even relatively low molecular weight, a number of models have been proposed to account for the transport of products of cytoplasmic protein synthesis into mitochondria, but convincing experimental support for these is not yet available. 

Table I summarizes data on the inhibition of synthesis of the individual outer-membrane proteins by the different antibiotics tested. In contrast to kasugomycin, tetracycline did not show clear differential inhibitory effects. On the other hand, chloramphenicol did reveal some differential inhibitory effects on the individual outer membrane protein synthesis, although no significant differential effects on overall envelope and cytoplasmic protein synthesis was detected (Fig. 3C). At concentrations of...

Fig. 3. Differential inhibition of envelope and cytoplasmic protein synthesis. The indicated amounts of kasugamycin (A), tetracycline (B), chloramphenicol (C), sparsomycin (D), and puromycin (E), were added to 15-ml cultures of E. coli MX74T2. The cultures were incubated at 37 C for 5 min and 25 of [ H]arginine was added. After the mixtures were incubated for another 1.5 min, the envelope and cytoplasmic fractions were prepared by differential centrifugation. In the case of rifampicin (F), the drug (200 Mg/ml) was added to 5 separate 15-ml cultures, and the mixtures were incubated for 2.5, 5, 10, 15, and 20 min, respectively. After the incubation, each culture was labeled with 25 iCi [ H]arginine for 1.5 minutes and envelope and cytoplasmic fractions were prepared. The control experiment was carried out without the addition of rifampicin. All data are expressed as rates of inhibition calculated as percentage of overall incorporation of the control (no addition of antibiotics). Envelope fraction,— — —. Cytoplasmic fraction, -0-0-. ...

---