A yeast ribosomal subunit association factor (AF) has been purified from a high-salt ribosomal wash. The purified enzyme is a thermostable protein that associates ribosomal subunits at low Mg2 + concentration without requiring energy. It appears to be an aggregate of trimers or dimers (molecular mass 125 or 79 kDa) which on sodium dodecyl sulfate gels shows the presence of a major protein band whose estimated molecular mass is 43 kDa. Evidence also indicates the existence of a 50-kDa polypeptide which seems to be unstable since with freezing and thawing it gives rise to the 43-kDa polypeptide. It was shown that the labelled factor interacts with 80s ribosomes and with 40s ribosomal subunits. The purified polypeptide reacts with antibodies directed against EF-la, this last protein recognizing the antibodies raised against AF. Likewise, both EF-la and AF associate ribosomal subunits in the same way. When EF-1 is heated, it not only maintains its association activity, but also behaves like a 43-kDa polypeptide in an SDS electrophoresis run. These observations strongly suggest that AF originates from EF-la, which implies that the well-known elongation factor may also play a role in the initiation step of protein synthesis.In the ribosomal cycle, when the ribosomal subunits are released at chain termination they may bind factors which prevent their reassociation and channel them into another cycle of synthesis, or they may recombine and enter the pool of monomers. In prokaryotic and eukaryotic cells, the monomer ribosomes accumulate at the expense of polysome degradation that takes place when protein synthesis diminishes under a variety of metabolically adverse conditions [l]. These single ribosomes can revert to polysomes when the rate of overall protein synthesis increases and in this way function as a reserve of potential subunits. This fluctuation in the rate of protein synthesis and the corresponding monomer level, which are not necessarily related to growth, may occur normally in eukaryotic cells. Such regulation may reflect variations in the activity of some factor or group of factors that participates either in the ribosomal subunit association or in the dissociation step. The interdependence between these two kinds of factors will determine whether subunits, released at peptide chain termination, will immediately reassociate with mRNA to initiate another round of synthesis or will couple and enter the pool of inactive monomers.Earlier studies have established that ribosomes are in dynamic equilibrium with their subunits [2-51. Vast amounts of information accumulated in the last two decades point to the existence of initiation factors that prevents the association of ribosomal subunits into single ribosomes but does not actively promote their dissociation. Dissociation of single ribosomes can take place spontaneously at low magnesium concentration, thereby generating subunits which are unable to revert to monosomes because binding of eIF-3 and eIF-4C to 40s subunits and binding of eIF-6 to 60s subunits block ...

An in vitro translation system has been prepared from Plasmodium falciparum by saponin lysis of infected-erythrocytes to free parasites which were homogeneized with glass beads, centrifuged to obtain a S-30 fraction followed by

The present investigation was conducted with an objective to validate the effect of the plant Azadirachta indica (neem) in human blood cells in a normoglycemic medium. This hypoglycemic effect had been attributed by the communities to this plant. In this study, aqueous extract of the plant was used in a concentration of 6.4% m/v and from this, following doses were prepared: 0.01 mg; 0.05 mg (low doses); 0.1 mg; 0.175 mg (medium doses); 0.7 mg and 1.4 mg (high doses); also it was used insulin Humulin R ® in different concentrations (0.1, 1, 10 nM) in order to compare both effects in a normoglycemic medium with human blood cells. The results demonstrated a hypoglycemic effect when the levels of glucose concentration went down in the normoglycemic medium in relation to the control. There was a significant effect (p<0.001) for the high doses group. These findings validate the hypoglycemic effect of this plant attributed by the communities.

No abstract

The polysomal mRNA from the cell-free system of the yeast Saccharomyces cerevisiae, in the absence of exogenous energy, binds to the 40s ribosomal subunit thus forming a 48s preinitiation complex which, with energy added, is converted into 80s initiation complex. By using ribosomes with a high affinity to polysomal mRNA (pmRNA) from an edeine-resistant mutant of S. cerevisiae in place of wild-type ribosomes, increased quantities of the 48s preinitiation complex are obtained.The pmRNA is found associated with several polypeptides having molecular masses of 11 5 -98 kDa, 72 kDa, 60 kDa and 51 kDa. These polypeptides, labelled with 1251, interact with 40s and 80s ribosomes and are essential for the formation of the 48s and 80s initiation complexes inasmuch as deproteinized pmRNA alone cannot initiate the process. In addition, other polypeptides present in the cytosol are required to carry out the abovementioned steps of protein synthesis.The existence of proteins associated with eucaryotic mRNA has been extensively documented [l -71. These complexes of RNA-protein, known as messenger ribonucleoprotein particles (mRNPs), are found in the nucleus, free in the cytosol (informosomes) or in the polysomal fraction. Free cytoplasmic mRNP has been suggested as being a possible precursor of the polysomal mRNP [8,9]. Indeed, evidence has been presented showing that proteins associated with mRNA in mouse L-cell polyribosomes interchange with free proteins present in the cytosol [lo]. mRNP complexes are composed of several polypeptides that have a wide range of molecular masses. Analyses of the protein composition of cytosol and polysomal mRNP fractions have revealed significant differences in their characteristic protein pattern, although a number of similar major proteins have been observed in both classes of mRNPs [l 1 -161. The presence of common proteins could be responsible for maintaining a defined structural organization of RNP that permits its playing a general role in mRNA metabolism, transportation and translation. The different polypeptides may represent elements of specificity for recruiting or potentiating certain messengers. Recently it has been reported that a 60-kDa messenger ribonucleoprotein exerts a positive control over the translation of mRNA in a cell-free system from embryo axes of dry pea seeds [17]. This type of control mechanism indicates that proteins complexed to polysomal RNP might be one of the keys to protein synthesis initiation.It has also been suggested that structural modifications exist between polypeptides of the cytoplasmic mRNP and polypeptides of the polysomes. This proposal could explain reports of the existence of cytoplasmic mRNPs which are nontranslatable [15, 18 -231. It is possible that specific proteins or modified polypeptides in the cytosol interact with mRNA in such a way that they prevent the entry of mRNA in the translation process: only when these protecting proteins dissociate the complex, may mRNA translation occur. Several reports agree with such an argument [24 -291. Th...