FIG. 3.

cysK is a trans-acting regulator of cysH expression. (A) Strains MCK530-DK1 (PcysH-lacZ, cysK thrC::Pxyl-cysK) and MCK530-C795 (PcysH-lacZ, cysK thrC::pDG795) were grown in sulfur-free-minimal medium supplemented with threonine, cystine, casein hydrolysate, yeast extract, and xylose. When cultures reached the exponential phase of growth, cells were recovered by centrifugation and washed several times with sulfur-free minimal media. The cells were then resuspended in sulfur-free minimal media supplemented with threonine and xylose in the absence or in the presence of cystine. Samples were taken 4 h after resuspension. β-Galactosidase activities are expressed in MU. (B) Cells were treated as described in panel A legend, but the experiment was carried out in the absence of xylose.