The growth rate control in Escherichia coli at near to maximum growth rates: the A-stat approach.

The growth characteristics of Escherichia coli K-12 in the continuous culture with a smooth increase in the dilution rate (A-stat) of various carbon sources (glucose, acetate, succinate, glycerol, lactate, acetate + succinate, casamino, acids + glucose) were studied. For all substrates studied the maximum value of specific respiration rate, QO2, remained between 14-18 mmol O2 h-1 g dwt-1 and the maximum growth rate varied from 0.22 h-1 on acetate to 0.77 h-1 on glucose + casamino acids. After the respiratory capacity of the cells was exhausted at growth rates mu > mu crit, the growth yield YXO2, increased slightly when the dilution rate increased. The maximum growth rate of Escherichia coli K12 was dependent on growth yield, respiratory capacity and glycolytic capacity of the strain. Analysis of the cultivation data using a stoichiometric flux model indicated that ATP synthesis in E. coli exceeds by two-fold that (theoretically) required to build up biomass. The experimental value of mATP < 4 mmol ATP h-1 g dwt-1 determined from A-stat cultivation data was low compared with the calculated 'unproductive hydrolysis' of ATP (64-103 mmole ATP g dwt-1).

In vivo deuteration of transfer RNAs: overexpression and large-scale purification of deuterated specific tRNAs.

Structural investigations of tRNA complexes using NMR or neutron scattering often require deuterated specific tRNAs. Those tRNAs are needed in large quantities and in highly purified and biologically active form. Fully deuterated tRNAs can be prepared from cells grown in deuterated minimal medium, but tRNA content under this conditions is low, due to regulation of tRNA biosynthesis in response to the slow growth of cells. Here we describe the large-scale preparation of two deuterated tRNA species, namely D-tRNAPhe and D-tRNAfMet (the method is also applicable for other tRNAs). Using overexpression constructs, the yield of specific deuterated tRNAs is improved by a factor of two to ten, depending on the tRNA and growth condition tested. The tRNAs are purified using a combination of classical chromatography on an anion exchange DEAE column with reversed phase preparative HPLC. Purification yields nearly homogenous deuterated tRNAs with a chargeability of 1400-1500 pmol amino acid/A260 unit. The deuterated tRNAs are of excellent biological activity.

Large-scale preparation of fully deuterated cell components. Ribosomes from Escherichia coli with high biological activity.

Some applications of NMR and of neutron scattering require fully deuterated biological material which should be highly active and available in large quantities. These requirements are hardly compatible since full deuteration is achieved easily only if cells are grown in minimal media. This condition used in standard batch fermentation results in both low yields and reduced activities of the biological mass. Here we report a method which combines the apparently incompatible requirements taking advantage of a recent observation according to which the appearance of growth inhibiting extracellular products could be prevented. The method was applied for growing Escherichia coli cells, strain MRE600rif (resistance against high doses of rifampicin is used as selection marker) on partially deuterated media (76% and 84% D2O) with glucose as carbon source and on deuterated acetate and succinate with 100% D2O when full deuteration was to be achieved. The essential point for preserving the log-phase character of the cells is that the cultivation is carried out at substrate limiting conditions thus keeping the growth rate at low levels (for glucose the growth rate, mu < or = 0.35 h-1, for acetate/succinate mu < or = 0.1 h-1) which avoids the accumulation of the substrate or of by-products in the medium. Our data suggest that acetate is a main extracellular component for accompanying or triggering the transition from logarithmic growth to stationary phase of E. coli cells cultivated on glucose as carbon source. The cells were first grown in fed-batch to high cell densities (above 50 g wet cells/l) under conditions of substrate limitations. A steady-flow fermentation followed keeping the growth rate at about mu of 0.1 h-1. Cells were harvested in kg quantities, the extracted ribosomes showed a normal complement of proteins, contained intact rRNA and were fully active. The ribosomal protein and rRNA fractions could be efficiently reconstituted to highly active particles. In the case of full deuteration a matching point of 120% (tentative D2O scale) was achieved. The reported method facilitates the preparation of deuterated biological material for applications in NMR and neutron scattering analysis.