FIG. 3.

Control of P_opimp by imp genes. Streptomyces strains TK64, Stm188 (expressing impA and impC), Stm189 (expressing impC), and Stm190 (expressing impA), harboring or not harboring plasmid pXE4 (containing the xylE gene) or pSJH6 (whose map is shown), as indicated, were grown on MY medium containing cellophane membranes and lysed by sonication, and expression of the xylE gene of the plasmids was measured in triplicate as indicated in Materials and Methods. The numbers for the XylE activity are the rate of change in optical density at 375 nm (OD₃₇₅) per minute per milligram of protein and are averages from three independent assays. “Expression” refers to the level of expression of the xylE gene in the plasmid tested compared to that in pXE4, containing a promoterless xylE gene. tsr, the thiostrepton resistance gene from Streptomyces spp.; bla, a β-lactamase gene from E. coli; ori, the ColE1 origin of replication; SCP2 rep, the SCP2 replication and stabilization functions (16); xylE, a promoterless copy of the xylE gene (18).