The suppression of defective translation by ppGpp and its role in the stringent response

Stringent control and protein synthesis in bacteria

Most bacteria have evolved a number of regulatory mechanisms which allow them to maintain a balanced and rather constant cellular composition in response to nutritional variations. In particular, when the availability of any aminoacyl-tRNA species becomes limiting (namely through amino acid starvation or inactivation of an aminoacyl-tRNA synthetase), several biochemically distinct physiological processes are significantly modified. This coordinate adjustment of cellular activity is termed the "stringent response". Under such conditions of aminoacyl-tRNA limitation, protein synthesis still proceeds, but various quantitative as well as qualitative changes in polypeptide metabolism can be observed. In this review, after a brief recall of the main characteristics of the stringent response, several aspects concerning protein synthesis in deprived bacteria have been presented. First, the rates of residual protein formation, peptide chain growth and protein degradation, and the molecular weight distribution of proteins newly synthesized have been analyzed. Then, the data relative to the biosynthetic regulation of non-ribosomal and ribosomal proteins have been summarized and compared to the results obtained from in vitro experiments using transcription-translation coupled systems. Finally, the problem of translational fidelity during deprivation has been discussed in connection with the metabolic behavior of polysomal structures which are still maintained in cells. The stringent dependence of cellular activity on aminoacyl-tRNA supply is known to be abolished by single-site mutations which confer to bacteria a phenotype referred to as "relaxed". These mutant strains provide an useful analytical tool in the scope of understanding the stringency phenomenon. Therefore, their proteosynthetic activity under aminoacyl-tRNA deprivation has also been studied here, in comparison to that of normal wild-type strains.

Protein synthesis results in guanosine-5'-diphosphate-3'-diphosphate synthesis in Escherichia coli minicells

Minicells of Escherichia coli DS410 synthesized guanosine-5'-diphosphate-3'-diphosphate and guanosine-5'-triphosphate-3'-diphosphate when synthesizing proteins in response to phage T7 infection. The guanosine-5'-diphosphate-3'-diphosphate synthesis was found to be relA+ dependent and inhibited by chloramphenicol.

Protein synthetic errors do not increase during aging of cultured human fibroblasts

To test the error catastrophe theory of aging we determined the error frequency of protein synthesis in several strains of cultured human fibroblasts at early and late passage. Error rates were calculated from analysis of native and substituted actins on two-dimensional gels of cellular proteins after induction of mistranslation by histidine starvation in the presence of histidinol. Early-passage cells from fetal, young, and old donors and cells from subjects with the Hutchinson-Gilford and Werner syndromes of accelerated aging had similar error frequencies. Late-passage cells from fetal, young, and old normal donors had similar or lower error frequencies than corresponding early-passage cells. No correlation was observed between error frequency, donor age, or maximal life span in vitro. We also examined an immortal cell line, simian virus 40-transformed W138 fibroblasts. These cells had a significantly elevated rate of mistranslation (2.8 +/- 0.2 x 10(-4))(+/- SEM) compared to their untransformed counterpart WI38 (0.6 +/- 0.1 X 10(-4)) or all diploid cells combined (1.1 +/- 0.1 x 10(-4)). Taken together, the data fail to support the error catastrophe theory of aging.

Protein identifications of O'Farrell two-dimensional gels: locations of 81 Escherichia coli proteins

The two-dimensional gel electrophoresis method of P. H. O'Farrell readily resolves approximately 1,000 proteins from whole-cell homogenates. We have found that the location of most individual proteins is sufficiently reproducible and precise to permit different laboratories to exchange information about them. We present the location of 81 Escherichia coli structural proteins, binding proteins, enzymes, and factors, identified with the aid of purified proteins supplied to us by many investigators.

Genetic heterozygosity in a natural population of Mus musculus assessed using two-dimensional electrophoresis

Since the introduction of gel electrophoresis to population genetics, estimates of genic heterozygosity at allozyme loci have been made for many organisms. However, attempts to extend the range of proteins surveyed have been hampered by concern that there may have been bias in the loci sampled: nearly all proteins for which population data are available are soluble in low-salt extracts and most are enzymes belonging to particular groups such as the nonspecific esterases and phosphatases, various dehydrogenases and the enzymes of the glycolytic pathways and the citric acid cycle. Until recently, there were no techniques for estimating protein variation which were without such bias. Two-dimensional gel electrophoresis, however, offered hope of overcoming this bias, and it has proved useful for the resolution of proteins in crude homogenates and for estimating genetic variation. We have examined heterozygosity in a wild population of the house mouse (Mus musculus) by means of two-dimensional electrophoresis of whole kidney. As we report here, the observed level (2%) is substantially below the level detected by starch gel electrophoresis. Our results corroborate similar differences observed in Drosophila and man.

"Two out of three" codon reading leading to mistranslation in vivo

Strains of Escherichia coli were starved for asparagine or lysine in order to increase the in vivo level of mistranslation. In a relA strain, asparagine starvation increased the error frequency in elongation factor Tu to 0.12 mistake per asparagine codon, while with lysine starvation in the same strain the error frequency per lysine codon was 0.008. The pattern of isoelectric point changes in the altered protein produced is consistent with third position misreading in the AAN codon group. This high level of mistranslation is not seen in streptomycin resistant (rpsL) strains or in most relA+ strains.

Genetic analysis of protein variations in Mus musculus using two-dimensional electrophoresis

We have investigated the variation of proteins from crude homogenates of mouse kidneys in several strains of Mus musculus by means of two-dimensional electrophoresis. In this study, we have used the strains C57BL/6J, DBA/2J, CD-1, M. m. castaneus, and M. m. molossinus, as well as offspring from crosses among these strains. Out of the 100 loci screened, we have found nine loci showing interstrain differences. We have found also that differences in the protein pattern as seen on two-dimensional gels are small among subspecies of Mus musculus.

Guanosine pentaphosphate and guanosine tetraphosphate accumulation and induction of Myxococcus xanthus fruiting body development

Development of multicellular fruiting bodies of Myxococcus xanthus can be induced by limitation of any of a number of different classes of amino acids. Investigated were amino acids that wild-type strains of M. xanthus are unable to synthesize (isoleucine, leucine, and valine), can synthesize at a low rate (phenylalanine), or can normally synthesize at an adequate rate (tryptophan and serine). In general, gradual rather than abrupt starvation for an essential amino acid was required for the induction of fruiting. Perhaps gradual starvation in general minimizes antagonism between amino acids present in the medium, as was documented for valine starvation. The previously reported induction of fruiting by a high concentration of threonine was shown to be specifically reversed by lysine. Threonine addition may starve cells for lysine by feedback inhibition of aspartokinase activity. Starvation for carbon-energy sources or inorganic phosphate also induced fruiting. As in other bacteria, amino acid starvation of M. xanthus leads to increases in cellular guanosine polyphosphate, usually consisting of large increases in the amount of guanosine pentaphosphate with smaller increases in the level of guanosine tetraphosphate. Guanosine polyphosphate accumulation is thus shown to be correlated with nutritional conditions that induce fruiting, and therefore may serve as an intracellular signal to trigger cells to end vegetative growth and initiate fruiting body development.

Mutation spoT of Escherichia coli increases expression of the histidine operon deleted for the attenuator

F'-episomes carrying the Salmonella typhimurium wild-type or attenuator-deleted histidine (his) operons were introduced into Escherichia coli strains containing relA or spoT single and double mutations known to affect guanosine 3'-diphosphate 5'-diphosphate (ppGpp) and guanosine 3'-triphosphate 5'-diphosphate (pppGpp) levels. Expression of the his operon and expression of the gene for 6-phosphogluconate dehydrogenase (gnd) were measured during balanced growth in amino acid-rich and minimal media. The data were consistent with the interpretation that ppGpp is a positive effector of his operon expression, whereas pppGpp is not an essential effector. The conclusion that his operon expression is maximally stimulated at a lower than maximum intracellular ppGpp concentration was further confirmed. Neither ppGpp nor pppGpp appeared to influence gnd gene expression. The metabolic regulation of the E. coli his operon was found to be similar to the ppGpp-meidated metabolic regulation of the S. typhimurium his operon.