Regulation of brome mosaic virus gene expression by restriction of initiation of protein synthesis

The translation of total and individual brome mosaic virus (BMV) RNAs was examined in a wheat germ cell-free system in the presence of various inhibitors. Inhibitors of the initiation of polypeptide synthesis, e.g., potassium ions, 7-methylguanosine 5′ -monophosphate, and aurintricarboxylic acid, were shown not only to inhibit overall BMV protein synthesis but also to change the ratio of BMV polypeptides synthesized. Under conditions restrictive for initiation, the translation of nonstructural BMV genes was suppressed, but coat protein synthesis proceeded at a high rate. A similar discrimination among BMV messengers was exerted by a regulatory protein kinase isolated from wheat germ. These results suggest that the regulation of the expression of BMV genes is based on a difference in the mechanism of formation of initiation complexes for individual BMV messages.     

Control of the yeast cell cycle by protein synthesis

The increased synthesis of ribosomal RNA (rRNA) is correlated with enhanced cell proliferation, and it has been suggested that rRNA metabolism may have a regulatory role in the progression of the cell cycle. Alternatively, it might be the ensuing more active protein synthesis that drives the cell cycle progression. We have found that treatment with low doses of cycloheximide dissociates rRNA and protein synthesis. In fact, after the addition of cycloheximide the protein synthesis rate is strongly inhibited, whereas the rate of rRNA synthesis is unaffected for some time. The progression of the cell cycle, monitored as analysis of DNA distribution by flow cytometry and as bud emergence, is quickly and largely inhibited, thus indicating that a sustained rRNA metabolism is not sufficient to allow continuous cycle progression. The effects of cycloheximide on the daughter and mother duplication times, on the mean cell volume, and on the volume at budding were also analyzed. The results suggest that protein synthesis, rather than rRNA synthesis, may have a key role in the control of cell cycle progression in Saccharomyces cerevisiae.     

Metabolism of c-myc gene products: c-myc mRNA and protein expression in the cell cycle.

The presence and synthesis of c-myc protein and mRNA in the cell cycle has been studied. We find that c-myc mRNA is present, at equivalent levels, at all times in the cell cycle with the possible exception of mitosis. Furthermore, we demonstrate that this mRNA is transcribed in both G1 and G2 phases. An analysis of the c-myc protein in vivo shows that de novo synthesis occurs in G1 and G2 and the protein turns over with a half-life of approximately 20-30 min in both phases. Furthermore, the level of c-myc protein rapidly increases in cell populations when they re-initiate the cell cycle, thereafter decreasing as the culture reaches quiescence. The results therefore suggest that expression of c-myc can be rapidly modulated and that it is activated during the G0 to G1 transition, but is expressed thereafter in the cell cycle.     

Control of tubulin and actin gene expression in Tetrahymena pyriformis during the cell cycle

Poly(A)-containing mRNA was isolated from division synchronized populations of the ciliated protozoan, Tetrahymena pyriformis. The level of tubulin and actin mRNA at specific cell cycle stages was analyzed by hybridization to tubulin and actin cDNA probes and by gel analysis of their in vitro translation products. The pattern of fluctuation of tubulin mRNA levels was similar to that observed for the in vivo tubulin synthesis previously reported [1]. This suggests that as the cells progress through the cell cycle, tubulin synthesis is controlled at the mRNA level. There was little fluctuation of actin synthesis or actin mRNA levels during the cell cycle, which may be indicative of a different regulatory mechanism for actin than for tubulin.     

Stop making sense: or Regulation at the level of termination in eukaryotic protein synthesis

An increasing number of examples of translational regulation at the level of termination has been recently reported in eukaryotes. This paper reviews our present knowledge on this topic and proposes an understanding of these regulations by relating the study of viral gene expression to a comprehensive view of the mechanisms and components of the translational process.     

Regulation of root hair initiation and expansin gene expression in Arabidopsis

The expression of two Arabidopsis expansin genes (AtEXP7 and AtEXP18) is tightly linked to root hair initiation; thus, the regulation of these genes was studied to elucidate how developmental, hormonal, and environmental factors orchestrate root hair formation. Exogenous ethylene and auxin, as well as separation of the root from the medium, stimulated root hair formation and the expression of these expansin genes. The effects of exogenous auxin and root separation on root hair formation required the ethylene signaling pathway. By contrast, blocking the endogenous ethylene pathway, either by genetic mutations or by a chemical inhibitor, did not affect normal root hair formation and expansin gene expression. These results indicate that the normal developmental pathway for root hair formation (i.e., not induced by external stimuli) is independent of the ethylene pathway. Promoter analyses of the expansin genes show that the same promoter elements that determine cell specificity also determine inducibility by ethylene, auxin, and root separation. Our study suggests that two distinctive signaling pathways, one developmental and the other environmental/hormonal, converge to modulate the initiation of the root hair and the expression of its specific expansin gene set.     

Regulation of cell-cycle initiation in yeast by nutrients and protein synthesis.

Arrested Saccharomyces cerevisiae cells initiate the cell cycle in an asynchronous mode. The asynchronous manner of cycle initiation generates variability in cell-cycle times of individual cells. Limiting concentrations of adenine, methionine or histidine regulate the rate of cycle initiation in auxotrophs. A sigmoidal curve of rate vs. concentration is obtained for each of the three substances. Moreover, the three curves have similar Hill coefficients of 2.4, suggesting that a common intermediate requiring adenine, methionine and histidine regulates cell-cycle initiation in yeast. Low concentrations of cycloheximide reduce the rate of cycle initiation of arrested cells that are released from the block in a similar way as limiting nutrients. It thus appears that the common intermediate that requires the limiting nutrients depends upon protein synthesis. The rate of cycle initiation is more sensitive to cycloheximide or nutrient limitation than is protein synthesis. It is also affected by limiting nutrients to a much greater extent than is the overall rate of protein accumulation (i.e., net protein synthesis). Hence the mechanism that controls cycle initiation does not depend on the overall synthesis or accumulation of proteins in the cell. It may depend on synthesis of particular proteins whose production or function requires the limiting nutrients. The high sensitivity of cycle initiation to a decrease in the rate of protein synthesis could explain the ability of yeast cells to complete the cycle and arrest at stationary phase upon depletion of medium components. The cells cannot initiate the cycle although their protein synthesis capacity remains sufficiently high to allow traversal of the rest of the cell cycle.