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

Abstract

The transport of l -methionine by the gram-positive species Brevibacterium linens CNRZ 918 is described. The one transport system ( K m = 55 μM) 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 α-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.