Nonsense alleles in the lacZ gene of E. coli do not completely eliminate enzyme activity as errors during protein synthesis allow some chains to be completed. The relative contributions of transcriptional and translational errors to this leakiness were investigated by two methods: the introduction of rho- alleles into extreme-polar mutants and the kinetics of beta-galactosidase induction. Virtually all the errors appeared to be transcriptional in the case of two extreme-polar and one non-polar mutation. These alleles should prove useful for further in vivo investigations of RNA polymerase accuracy. With two other non-polar alleles, transcriptional mistakes were low and translational ones high. The frequency of RNA polymerase errors was context-dependent and varied for different nonsense codons in the same position and for the same codon in different positions. The reasons why some alleles showed no activity due to translational errors could not be clearly established. However, increasing the rates of ribosomal errors from one such allele with streptomycin raised the contribution of ribosomal errors to activity markedly and non-linearly. Translational mistakes may give rise to active enzyme only if the monomers are formed at a rate sufficient for effective aggregation to the normal tetramer.
SUMMARY:Streptococcus fuecalis has been grown anaerobically in continuous culture on a defined medium. Under these conditions it is possible to maintain (a) the glucose concentration, (b) the tryptophan concentration, and (c) the generation time a t predetermined values, and to study the effects of changes in these variables on the culture. When growth was limited by the glucose supply, the glucose yield constant (dry weight of cells €'ormed/weight of glucose used) was greater than when tryptophan was limiting. The glucose yield constant under conditions of tryptophan limitation progressively fell with increasing supplies of glucose until it reached a minimum value. Under conditions of tryptophan limitation and large excess of glucose, the amount of glucose used per unit weight of cells per unit time remained roughly constant irrespective of growth rate. It is concluded that the needs of cell synthesis in S. fueculis do not control the rate of glucose breakdown, i.e. the rate of the energy-yielding metabolism. At slow growth rates the end products of glucose metabolism included volatile fatty acids, which were not present, or present in small amounts, a t faster growth rates.In the course of his studies of the factors which control the growth of bacteria Monod (1942) established that, when the energy source is the limiting nutrient, the quotient-weight of cells produced/weight of energy source used-is constant irrespective of the concentration of the carbon source. This quotient is referred to as the growth yield constant (GYC). Monod also showed that there is a simple relationship between the specific growth rate and the concentration of the growth-limiting nutrient. The saturation constant, i.e. the concentration of the limiting nutrient at which the specific growth rate is half maximal, is very low and Monod obtained values of 4 , 2 and 25 mg/l. for glucose, mannitol and lactose respectively, as carbon sources for Escherichia coli.In all, Monod studied three organisms, Escherichia coli, Bacillus subtilis and Salmonella typhimurium. Monod's findings have been repeatedly confirmed by other microbiologists using other organisms grown batchwise upon a wide range of energy sources and the constancy of the GYC of a given organism grown on a particular energy source can be taken as established. However, it is important to realize that the values obtained by the various workers apply solely to the particular conditions employed, i.e. to growth in batch culture with the substrate in question the factor limiting growth and all other essential nutrients present in excess, It is clear from what has been said above that, under these conditions, the specific growth rate is maximal for almost the whole period of growth.
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