Inhibition of KupA and KupB transport activity by c-di-AMP. E. coli LB2003 carrying the relevant plasmids was cultivated in minimal salt M9mod medium, supplemented with 2.5 μM IPTG for kup gene induction and without arabinose (left) or with 0.005% l-arabinose (right) for induction of the cdaA alleles. (A and B) The strain carried the plasmid combinations pIQ309 (KupA)/pBP370 (CdaA), pIQ309 (KupA)/pBP373 (CdaA*), or pWH844/pBAD33 (empty vector control). (C and D) The strain carried the plasmid combinations pIQ310 (KupB)/pBP370 (CdaA), pIQ310 (KupB)/pBP373 (CdaA*), or pWH844/pBAD33 (empty vector control).