ROLE OF THE GENETIC MESSAGE IN POLYRIBOSOME FUNCTION

Hyaluronate-proteoglycan complex: evidence for separate biosynthesis mechanisms of the macromolecules

In cartilage cells biosynthesis of hyaluronate and chondroitin-4-,-6-sulfate from proteoglycan takes place via two different distinct precursor pools; the synthesis of hyaluronate appears to require the unaffected formation of nucleotides and nucleic acids, whereas that of proteoglycan is very sensitive to the modulation of protein biosynthesis.

An investigation of mistranslation in vivo induced by streptomycin by an examination of the susceptibility of abnormal proteins to degradation

Proteolysis rates in vivo were measured in Escherichia coli cultures during treatment with dihydrostreptomycin and under various other conditions. Dihydrostreptomycin treatment caused an increase in the proteolysis rate, compared to untreated controls. The proteolytic system in vivo responsible for the elevated proteolysis in the early stages of dihydrostreptomycin treatment, or that during canavanine and puromycin treatment, were not inhibited by addition of phenylmethanesulphonyl fluoride. This agent did inhibit proteolysis rates in cultures whose growth was inhibited by starvation, or had been completely stopped by dihydrostreptomycin. It seems, therefore, that the extremely high proteolysis rates in cultures at this stage of dihydrostreptomycin treatment were due to the action of two protease systems: the one concerned with the breakdown of abnormal proteins, and the other concerned with normal protein turnover and active during a non-specific decline of growth. The proteolytic rate at complete growth inhibition brought about by dihydrostreptomycin was intermediate between those induced by canavanine and puromycin at the same stage of treatment. This indicated a similar hierarchy in the extent and nature of abnormality in the proteins synthesized under these conditions. The relationship between the abnormality of proteins induced by dihydrostreptomycin and the importance of this in the antibiotic mechanism is discussed.

Degradation of abnormal proteins in HeLa cells

The experiments show that abnoramal proteins are degraded faster than normal ones in HeLa cells. Among the fragmentary proteins made in the presence of puromycin, those with low molecular weight are least stable. Proteins made after incubation with 5-fluorouracil or in the presence of some amino acid analogues are also unstable. Breakdown of proteins made in the presence or absence of puromycin is nearly unaffected by cycloheximide and is independent of pH between 7 and 8.

Control of haemoglobin synthesis. The effects of iron deprivation, cobalt and temperature on the rate and extent of globin synthesis in reticulocytes

A detailed examination of the kinetics of protein synthesis in rabbit reticulocytes in the presence of the iron chelating agent 2,2'-dipyridyl showed that between 30 degrees C and 42 degrees C there were characteristically two distinct phases of protein synthesis. An initial phase (I), in which no inhibition of protein synthesis was apparent, was followed by a gradual decline in the rate of protein synthesis leading to the second phase (II) in which protein synthesis occurred at a linear but inhibited rate for extended periods. In contrast, below 30 degrees C, incubation in the presence of dipyridyl caused no inhibition of protein synthesis. Between 30 degrees C and 42 degrees C the duration and amount of protein synthesis occurring in phase I before the onset of inhibition were inversely related of the inhibition as was the final rate of incorporation in phase II. During phase II, a partial reversal of the inhibition caused by dipyridyl was obtained by lowering the incubation temperature. This resulted in a burst of protein synthesis at the uninhibited rate until the amount of protein synthesis reached the same level as that in reticulocytes maintained continuously with dipyridyl at the lower incubation temperature. This burst of synthesis was observed in reticulocytes which had been held in phase II for as long as 90 min. It was also possible to reverse the inhibition by addition of haemin to cells in phase II. At any particular incubation temperature, a fixed number of rounds of protein synthesis had to occur before the onset of phase II became apparent. By the use of puromycin we showed that this was not a requirement for the synthesis of globin or of any other protein. We believe that this critical amount of protein synthesis reflects the residual ability of reticulocytes to initiate new protein chains in the absence of concurrent haem synthesis. Reticulocytes preincubated in the presence of cobaltous ions showed almost no inhibition of protein synthesis upon subsequent incubation with dipyridyl. The results are compared to those obtained in reticulocyte lysates and are discussed in terms of current theories to account for control of protein chain initiation by haemin.

Membrane-bound ribosomes of myeloma cells. III. The role of the messenger RNA and the nascent polypeptide chain in the binding of ribosomes to membranes

Mild ribonuclease treatment of the membrane fraction of P3K cells released three types of membrane-bound ribosomal particles: (a) all the newly made native 40S subunits detected after 2 h of [3H]uridine pulse. Since after a 3-min pulse with [35S]methionine these membrane native subunits appear to contain at least sevenfold more Met-tRNA per particle than the free native subunits, they may all be initiation complexes with mRNA molecules which have just become associated with the membranes; (b) about 50% of the ribosomes present in polyribosomes. Evidence is presented that the released ribosomes carry nascent chains about two and a half to three times shorter than those present on the ribosomes remaining bound to the membranes. It is proposed that in the membrane-bound polyribosomes of P3K cells, only the ribosomes closer to the 3' end of the mRNA molecules are directly bound, while the latest ribosomes to enter the polyribosomal structures are indirectly bound through the mRNA molecules; (c) a small number of 40S subunits of polyribosomal origin, presumably initiation complexes attached at the 5' end of mRNA molecules of polyribosomes. When the P3K cells were incubated with inhibitors acting at different steps of protein synthesis, it was found that puromycin and pactamycin decreased by about 40% the proportion of ribosomes in the membrane fraction, while cycloheximide and anisomycin had no such effect. The ribosomes remaining on the membrane fraction of puromycin-treated cells consisted of a few polyribosomes, and of an accumulation of 80S and 60S particles, which were almost entirely released by high salt treatment of the membranes. The membrane-bound ribosomes found after pactamycin treatment consisted of a few polyribosomes, with a striking accumulation of native 60S subunits and an increased number of native 40S subunits. On the basis of the observations made in this and the preceding papers, a model for the binding of ribosomes to membranes and for the ribosomal cycle on the membranes is proposed. It is suggested that ribosomal subunits exchange between free and membrane-bound polyribosomes through the cytoplasmic pool of free native subunits, and that their entry into membrane-bound ribosomes is mediated by mRNA molecules associated with membranes.

Transport of messenger RNA into different classes of membrane-associated polyribosomes in Ehrlich-ascites-tumor cells

The membrane-bound polyribosomes in Ehrlich ascites tumor cells can be separated into a loosely bound and a tightly bound fraction by means of a high salt treatment. Both membrane fractions as well as the free polyribosomes in the supernatant synthesize about the same set of proteins, suggesting a close relationship between these polyribosome fractions in the Ehrlich cell. Relatively high concentrations of cycloheximide do not prevent newly synthesized poly(A)-containing mRNA from entering the tightly bound polyribosome fraction. Nor had treatment of the cells with puromycin in the presence of cycloheximide, which released about 70% of the nascent chains, any significant effect on the entrance of newly synthesized mRNA into tightly bound polyribosomes. These results suggest that in ehrlich ascites tumor cells nascent polypeptide chains are not involved in the binding of polyribosomes to membranes.

Dissociation of mammalian polyribosomes into subunits by puromycin

Hepatic ribosomes have been dissociated into biologically active subunits as follows. Polysomes were treated at 0 degrees C with puromycin at high ionic strength. This released most of the nascent polypeptide chains without dissociating the polysomes, which retained the mRNA and the tRNA moiety of peptidyl tRNA, but were unable to continue the translation of mRNA. The polysomes were then heated to 37 degrees C, when they dissociated completely into subunits. Similar treatment without puromycin resulted in only partial dissociation.

Foot-and-mouth disease virus-induced alterations of baby hamster kidney cell macromolecular biosynthesis: inhibition of ribonucleic acid methylation and stimulation of ribonucleic acid synthesis

The kinetics of ribonucleic acid (RNA) and protein synthesis and RNA methylation were examined after foot-and-mouth disease virus (FMDV) infection of baby hamster kidney cells. The synthesis of RNA extracted from the whole cells was stimulated two- to threefold above the control level of synthesis. This increased rate was attributed to viral RNA synthesis. The inhibition of host RNA methylation was concomitant with but more pronounced than protein synthesis inhibition. The methylation of transfer RNA was initially inhibited by virus infection, but rose to within 70 to 80% of the control level just prior to the production of maximal amounts of virus-specific RNA polymerase. Cycloheximide studies showed that rapid cessation of protein synthesis did not result in the immediate cessation of RNA methylation. A comparison between the kinetics of inhibition of these processes by cycloheximide and FMDV infection suggests that FMDV selectively inhibits RNA methylation.