- Volume 139, Issue 5, 1993

5-Methylthioribose (MTR) kinase catalyses a key step in the recycling of methionine from 5′-methylthioadenosine, a co-product of polyamine biosynthesis, in Klebsiella pneumoniae. In defined medium lacking methionine, K. pneumoniae exhibits abundant MTR kinase activity. When the bacterium is transferred to a medium containing 10 mm-methionine, the specific activity of MTR kinase decreases in a fashion consistent with repression of new enzyme synthesis and dilution of existing enzyme by cell division. The specific activity of methionine synthase decreases to a similar degree under the same conditions. In Escherichia coli and Salmonella typhimurium, the gene for methionine synthase is co-ordinately controlled as part of the methionine regulon. Taken together, our results indicate that a methionine regulon may function in K. pneumoniae and that expression of MTR kinase may be under its control.

A mutant (strain LLD-15) of Bacillus stearothermophilus strain LLD-R, that lacks l-lactate dehydrogenase activity was studied in anaerobic single-stage continuous culture using a defined minimal medium for its capacity to produce ethanol from sucrose at 70 °C. Cultures were stable at neutral pH, dilution rates below 0·25 h−1 and sucrose concentrations below 15 g l−1, producing mainly ethanol, CO2, formate and acetate plus a little succinate. The data obtained were used to estimate the maintenance energy coefficient at 70 °C and showed that the relative fluxes through the pyruvate-formate lyase (PFL) pathway and an anaerobic pyruvate dehydrogenase (PDH) pathway were about equal, though the later predominates at low dilution rates. At higher dilution rates or at acid pH, l-lactate became the major anaerobic product due to takeover by wild-type revertants. Reversion occurred only above a critical specific sucrose consumption rate of 4·2 g sucrose h−1 (g cells)−1 at pH 7, at which point pyruvate production was also seen. This can be attributed to saturation of the PFL pathway and PDH pathways by increasing glycolytic flux and consequent accumulation of intracellular pyruvate. The growth rate then declines, but a small population of revertant LLD-R cells would always be present in the bioreactor since the mutation in the l-lactate dehydrogenase gene has a significant reversion rate. These revertants have no selective advantage below the critical sugar uptake rate, but at higher rates they divert excess pyruvate to L-lactate, so their growth is unimpaired and they take over in the culture. At lower pH, growth restriction and reversion occur at low sucrose consumption rates due to difficulty in excreting acetic and formic acids against a pH gradient.

Summary: Compounds (e.g. trans-isohumulone) derived from flowers of the hop plant (Humulus lupulus L.) protect beer from spoilage. Cells of Lactobacillus brevis IFO 3960 did not die when they were exposed to 40 μM-trans-isohumulone for up to 120 h. At higher concentrations (80, 120 μM) death occurred after a lag period of about 30 h. trans-Isohumulone dissipated the transmembrane pH gradient of non-growing cells and reduced their ability to accumulate L-[U-14C]leucine. The membrane potential was dissipated to a smaller extent. Addition of trans-isohumulone to cells that had accumulated L[U-14C]leucine, under conditions in which no synthesis of protein took place, caused slow leakage of radiolabelled leucine. trans-Isohumulone did not inhibit the activity of the proton-translocating membrane ATPase. Potentiometric experiments with resting cell suspensions suggested that trans-isohumulone acted as an ionophore of the mobile-carrier type, causing electroneutral exchange of H+ for divalent cations such as Mn2+. A second monovalent cation (e.g. K+) was essential for protonophoric activity.

The response of 31 ascomycetous yeasts to a reduction in water activity (a w) adjusted with d-glucose or NaCl was investigated. The growth of most yeasts was more tolerant to glucose than to NaCl at equivalent a w. Zygosaccharomyces rouxii was the most osmotolerant yeast examined. Natural abundance 13C-NMR spectroscopy and HPLC analyses of eight yeasts indicated that glycerol and arabitol or mannitol were accumulated intracellularly in response to a w. reduction. Pichia sorbitophila, Candida cacaoi, Candida magnoliae and Zygosaccharomyces bisporus responded to reduced a w by a decrease in specific growth rate and cell volume, and an accumulation of glycerol. The other polyol accumulated did not increase in concentration with a w reduction to the same degree as glycerol. A polyol concentration ratio (intra/extracellular) as high as 800-fold was attained across the membrane. Greater amounts of polyols were produced at equivalent a w values when adjusted with glucose than with NaCl. The ability to accumulate high concentrations of polyols appears to be the most important criterion in determining osmotolerance.