FIG. 7.

(A) Comparison of the acetic acid-induced inhibition of external medium acidification rate promoted by energized cells of the parental strain BY4741 (□) and of the deletion mutant Δhrk1 (○). Deenergized cell suspensions of mid-exponential phase cells of both strains were either exposed or not to acetic acid (0 or 1.2 mM at pH 4.0) and then energized with a pulse of glucose (2%) to assess plasma membrane H⁺-ATPase activity, associated to the external acidification rate of the growth medium. The values shown are means of, at least, three independent experiments that gave rise to similar results. (B) Time course of the accumulation ratio of [1-¹⁴C]-acetic acid (corresponding to the ratio between intracellular (Int) and extracellular (Ext) [1-¹⁴C]-acetic acid) during cultivation of cells of the parental strain BY4741 (□) or the Δhrk1 mutant (○) in MM4 growth medium supplemented with 60 mM acetic acid (at pH 4.0). Cells used to perform the assay were grown in unsupplemented MM4 medium (at pH 4.0), harvested by centrifugation and reinoculated in fresh growth medium supplemented with radiolabeled and cold acetic acid as described in Materials and Methods.