FIG 8.

Coregulation of Pho89 and Ena1 expression allows functional coupling between both proteins under conditions of high-pH stress. (A) Concerted coregulation of Pho89 (open circles) and Ena1 (closed circles) expression. Shown are time courses for ENA1 and PHO89 mRNA accumulation (top) or protein accumulation expressed as a percentage over the maximum signal (bottom) after shifting cells from pH 5.5 to 8.0 (see Materials and Methods for details). (B) The indicated strains at an OD₆₆₀ of 0.05, plus a 10-fold dilution, were spotted onto YNB-based medium (lacking phosphate and sodium) agar plates supplemented with 5 mM NaCl and the indicated amounts of potassium phosphate and adjusted to different pHs. Growth was monitored after 3 days. (C) The indicated strains were shifted for 1 h to low-P_i (0.2 mM) medium at pH 5.5 and then shifted to pH 7.8, always in the presence of 5 mM NaCl. Samples were taken at the indicated periods and processed for determination of the intracellular Na⁺ content. Data are means ± standard errors of the means from 4 to 8 independent experiments. (D) Illustration of the proposed functional link between Ena1 and Pho89 under high-pH stress (see the text for explanations).