Methionine transport in Yersinia pestis

Na-Stimulated Transport of l-Methionine in Brevibacterium linens CNRZ 918

The transport of l-methionine by the gram-positive species Brevibacterium linens CNRZ 918 is described. The one transport system (K(m) = 55 muM) found is constitutive for l-methionine, stereospecific, and pH and temperature dependent. Entry of l-methionine into cells is controlled by the internal methionine pool. Competition studies indicate that l-methionine and alpha-aminobutyric acid share a common carrier for their transport. Neither methionine derivatives substituted on the amino or carboxyl groups nor d-methionine was an inhibitor, whereas powerful inhibition was shown by l-cysteine, s-methyl-l-cysteine, dl-selenomethionine and dl-homocysteine. Sodium plays important and varied roles in l-methionine transport by B. linens CNRZ 918: (i) it stimulates transport without affecting the K(m), (ii) it increases the specific activity (on a biomass basis) of the l-methionine transport system when present with methionine in the medium, suggesting a coinduction mechanism. l-Methionine transport requires an exogenous energy source, which may be succinic, lactic, acetic, or pyruvic acid but not glucose or sucrose. The fact that l-methionine transport was stimulated by potassium arsenate and to a lesser extent by potassium fluoride suggests that high-energy phosphorylated intermediates are not involved in the process. Monensin eliminates stimulation by sodium. Gramicidin and carbonyl cyanide-m-chlorophenylhydrazone act in the presence or absence of Na. N-Ethylmaleimide, p-chloromercurobenzoate, valinomycin, sodium azide, and potassium cyanide have no or only a partial inhibitory effect. These results tend to indicate that the proton motive force reinforced by the Na gradient is involved in the mechanism of energy coupling of l-methionine transport by B. linens CNRZ 918. Thus, this transport is partially similar to the well-described systems in gram-negative bacteria, except for the role of sodium, which is very effective in B. linens, a species adapted to the high sodium levels of its niche.

Consequences of aspartase deficiency in Yersinia pestis

Growing cells of Yersinia pseudotuberculosis, but not those of closely related Yersinia pestis, rapidly destroyed exogenous L-aspartic and L-glutamic acids, thus prompting a comparative study of dicarboxylic amino acid catabolism. Rates of amino acid metabolism by resting cells of both species were determined at pH 5.5, 7.0, and 8.5. Regardless of pH, Y. pseudotuberculosis destroyed L-glutamic acid, L-glutamine, L-aspartic acid, and L-asparagine at rates greater than those observed for Y. pestis. Although rates of proline degardation were similar, its metabolism by Y. pestis at pH 8.5 resulted in excretion of glutamic and aspartic acids. Similarly, Y. pestis excreted aspartic acid when incubated with L-glutamic acid (pH 8.5) or L-asparagine (pH 5.5, 7.0, and 8.5). Aspartase activity was not detected in extracts of 10 strains of Y. pestis but was present in all 11 isolates of Y. pseudotuberculosis. The latter contained significantly more glutaminase, asparaginase, and L-glutamate-oxalacetate transminase activity than did extracts of Y. pestis; specific activities of L-glutamate dehydrogenase and alpha-ketoglutarate dehydrogenase were similar. The observed differences in dicarboxylic amino acid metabolism are traceable to asparatase deficiency in Y. pestis and may account for the slow doubling time of this organism relative to Y. pseudotuberculosis.

Transport and utilization of D-methionine and other methionine sources in Escherichia coli

The transport and utilization of D-methionine was investigated in several strains of Escherichia coli K-12. Wild-type cells exhibit a single transport system with a Km of 1.16 muM. This activity exhibits a specificity similar to that of the uptake of L-methionine. The activity toward the D-isomer and the high-affinity uptake of L-methionine are lost in strains mutant in metD, along with the ability to utilize D-methionine as methionine source. Both activities respond identically to gene dosage of metD and are both restored in revertants or transductants. However, although L-methionine is a potent inhibitor of D-methionine uptake, D-methionine has little or no effect on the uptake of the L-isomer. No mutants altered in the uptake of only one of the two isomers were found in a screening. Regulation of both activities was similar in their response to the internal methionine pool, and some evidence was suggestive of partial repressive control of these activities. The evidence is most consistent with the role of the metD product as a common step for two methionine-specific uptake systems, but other gene products may represent the initial substrate binding sites. This system also appears to be involved in the uptake of N-acetyl methionine and methionine sulfoxide and methionine sulfoximine. The uptake of the keto analogue of methionine, alpha-keto-gamma-methiol butyrate, appears to be mediated by a separate system specific for alpha-keto straight-chain acids 5- to 6-carbon units in length.