Growth rate dependence of transfer RNA abundance in Escherichia coli.

We have tested the predictions of a model that accounts for the codon preferences of bacteria in terms of a growth maximization strategy. According to this model the tRNA species cognate to minor and major codons should be regulated differently under different growth conditions: the isoacceptors cognate to major codons should increase at fast growth rates while those cognate to minor codons should decrease at fast growth rates. We have used a quantitative Northern blotting technique to measure the abundance of the methionine and the leucine isoacceptor families over growth rates ranging from 0.5 to 2.1 doublings per hour. Five tRNA species that are cognate to major codons (tRNA(eMet), tRNA(1fMet), tRNA(2fMet), tRNA(1Leu) and tRNA(3Leu) increase both as a relative fraction of total tRNA and in absolute concentration with increasing growth rates. Three tRNA species that are cognate to minor codons (tRNA(2Leu), tRNA(4Leu) and tRNA(5Leu) decrease as a relative fraction of total RNA and in absolute concentration with increasing growth rates. These data suggest that the abundances of groups of tRNA species are regulated in different ways, and that they are not regulated simply according to isoacceptor specificity. In particular, the data support the growth optimization model for codon bias.     

Linear Cell Growth in Escherichia coli

Growth was studied in synchronous cultures of Escherichia coli, using three strains and several rates of cell division. Synchrony was obtained by the Mitchison-Vincent technique. Controls gave no discernible perturbation in growth or rate of cell division. In all cases, mean cell volumes increased linearly (rather than exponentially) during the cycle except possibly for a small period near the end of the cycle. Linear volume growth occurred in synchronous cultures established from cells of different sizes, and also for the first volume doubling of cells prevented from division by a shift up to a more rapid growth rate. As a model for linear kinetics, it is suggested that linear growth represents constant uptake of all major nutrient factors during the cycle, and that constant uptake in turn is established by the presence of a constant number of functional binding or accumulation sites for each growth factor during linear growth of the cell.     

Platinum-Induced Filamentous Growth in Escherichia coli

Certain group VIIIB transition metal compounds were found to inhibit cell division in Escherichia coli, causing marked filamentous growth. Gram-negative bacilli were the most sensitive to this effect, whereas gram-positive bacilli responded only at near-toxic levels of the metal. None of the cocci tested showed any apparent effect. Cytokinesis (cross-septation) can be initiated by removal or decrease of platinum, but not by treatment with pantoyl lactone, divalent cations, or a temperature of 42 C.     

Transient regulation of enzyme synthesis in Escherichia coli

Summary After lysine addition to an exponentially growing culture of Escherichia coli K12, the kinetics of repression of aspartokinase III synthesis show a transient regulatory phenomenon: during one generation, enzyme synthesis is practically equal to zero (Fig. 1). A similar phenomenon appears to be involved during repression of aspartokinase I-homoserine dehydrogenase I synthesis by threonine and isoleucine (Fig. 2). This sort of phenomenon has been previously reported in another system and interpreted as an indication of regulation at the translational level.