Figure 4.

Transcriptional regulation of the ccdAB_ST TA system. The 300 bp region upstream to ccdA_ST was cloned as a transcriptional fusion with a promoter-less lacZ reporter gene in plasmid pMP220. (A) β-galactosidase was measured in different genetic backgrounds of S. Typhimurium SV5015 transformed with pPccdAB-lacZ: (i) pSLT wild type, (ii) a strain cured of pSLT, (iii) a pSLT plasmid deficient for ccdAB_ST TA system (including the promoter of the system), (iv) a pSLT lacking the toxin ccdB_ST gene, and (v) a pSLT in which the PccdAB_ST promoter was eliminated. Absence of TA complexes releases the CcdAB_ST transcriptional repression, resulting in increasing β-galactosidase activity (B) Wild type S. Typhimurium SV5015 was transformed with pPccdAB-lacZ and either a plasmid expressing the non-functional copies of CcdB_ST or VapC_ST to further measure β-galactosidase activity. Cultures were grown to OD₆₀₀ of 0.3 and then expression of ccdB_ST or vapC_ST genes was induced by adding 0.3% arabinose during 1 h. Excess of CcdB_ST specifically shows conditional cooperativity effect as its overexpression derepresses transcription at PccdAB_ST promoter of PSLT plasmid. Data represent the means and standard deviations from four independent experiments. ^***P < 0.001 by one-way ANOVA.