pstS expression in S. lividans TK24 (wt) and the phoP mutant. A) Semiquantitative RT-PCR analysis of pstS and of phoP expression in the wild-type (TK24) and the phoP mutant (ΔphoP). RNAs from 60 h-cultures were used. The size of the amplified bands is 530 bp for pstS, 327 bp for phoP, and 416 bp for the 16S RNA. Forty amplification cycles were performed for the phoP gene in both strains and for the pstS gene in the phoP mutant. 20 amplification cycles were done for the rest of the RT-PCRs. B) Coomassie-Blue-R-stained SDS-PAGE of S. lividans TK24 and ΔphoP supernatants showing the production of the Xys1 xylanase under the control of the pstS promoter with the PHO boxes deleted (pNUF11). pNX30 and pNUF5 were used as controls (10 μl of 4-day-old culture supernatants were loaded per lane).