3′, 5′cAMP promotes Clr binding to the Clr-box at the smc02178 promoter. (A) smc02178-lacZ expression was monitored ex planta in S. meliloti 1021 WT and a Clr-box deleted strain (TGΔCA) after addition of 3′, 5′cAMP. (B, C) EMSA assays showing Clr-GST binding to 28-mers oligomers carrying the WT Clr-box (B) or a mutated version (C) (see Additional file 10). Assays were performed in the presence of 1.75 nM oligomers, 200 μM 3′, 5′cAMP, and varied amounts of Clr (35 μM, 17.5 μM, 8.75 μM, 3.5 μM and 1.75 μM). See methods for details.