Influence of Environment on the Content and Composition of Microbial Free Amino Acid Pools

Abstract

The free amino acid pool contents of Gram-negative bacteria (Aerobacter aerogenes, Erwinia carotovora, Pseudomonas fluorescens) were studied as functions of the growth environment and were compared with those from correspondingly grown cultures of Gram-positive bacteria (Bacillus subtilis var. niger, B. megaterium, B. polymyxa) and the yeast Saccharomyces cere visiae.Although the pools of the Gram-positive bacteria and the yeast contained five to 20 times the concentration of free amino acids present in the pools of Gram-negative bacteria, all pools were similar in containing only a limited range of detectable amino acids. Glutamate invariably predominated and generally accounted for over 50 % of the total amino acid content of the pool.The contents and composition of pools from micro-organisms maintained in steady states in chemostat cultures did not vary with time, but changed significantly with changes in either growth rate or the nature of the growth limitation. However, these pool variations were small compared with those resulting from addition of 2 % (w/v) NaCl to a culture of growing bacteria.With cultures of Gram-negative bacteria, sudden changes in medium salinity effected marked and rapid changes in free glutamate content; with Grampositive bacteria, similar changes occurred, but extremely slowly. Addition of 4 % (w/v) NaCl to growing yeast cultures brought about no observed changes in pool size or composition. These results are discussed with reference to the involvement of free amino acids in synthesis and functioning of microorganisms.

Response of Rhizobium fredii P220 to osmotic shock: Interrelationships between K+, Mg2+, glutamate and homospermidine

“…The response of non-growing cells of strain P220 to the short-term downshock or upshock in the present experiment (Figs 1 and 3) implies that homospd and Mg", together with I<+ and glutamate, are the cellular components closely associated with osmoregulation in R. There is strong evidence that K+ plays a major role in bacterial osmoregulation, as well as in maintaining the structure of ribosomes (Epstein, 1986). It has also been established that I<+ uptake under salt stress accompanies the accumulation of cellular glutamate as the counteranion in several bacterial species, including Rhixobim (Tempest & Meers, 1970;Measures, 1975;Hua e t al., 1982;Yelton e t a!., 1983;Cayley e t a/., 1989). Measures (1 975) reported that high concentrations of K+ enhanced the enzyme activity of glutamate dehydrogenase in cellfree systems prepared from several bacterial species, implying a possible relationship between I<+ influx and enlargement of the glutamate pool size during osmotic upshock.…”

The L box regulon: Lysine sensing by leader RNAs of bacterial lysine biosynthesis genes

“…In organisms that use DAP or lysine as a cell wall component, it is essential that this pathway be sufficiently active to provide adequate levels for both cell wall biosynthesis and protein synthesis. The intracellular pool of free lysine in B. subtilis has been reported at 2 mM during growth in the absence of exogenous lysine, conditions under which lysine biosynthesis genes are expressed (31). The requirement for 3 mM lysine for efficient transcription termination in vitro is therefore within a physiologically relevant range.…”

Glu-tRNA Gln amidotransferase: A novel heterotrimeric enzyme required for correct decoding of glutamine codons during translation

“…Alternatively, the larger framework of an aminoacylated tRNA may be necessary for discrimination between Glu and Gln for proper tRNA charging (3). Either of these two scenarios are plausible because several organisms, the Grampositive bacteria in particular, which use Glu-AdT, have unusually elevated glutamic acid concentrations (31).…”