CupAR negatively controls the key protein CupA in the carbon acquisition complex NDH-1MS in Synechocystis sp. PCC 6803.

The low CO-inducible NDH complex (NDH-1MS) plays a crucial role in the cyanobacterial CO-concentrating mechanism. However, the components in this complex and the regulation mechanism are still not completely understood. Using a mutant only with NDH-1MS as active Ci sequestration system, we identified a functional gene sll1736 named as cupAR (CupA Regulator). The cupAR deletion mutant, ΔcupAR, grew faster than the WT under high CO (HC) condition, more evidently at low pH. The activities of O evolution, CO uptake，NDH-1, and the building up of a transthylakoid proton were stimulated in this mutant under HC conditions. The cupAR gene is cotranscribed with the NDH-1S operon (ndhF3-ndhD3-cupA) and encoded protein, which specifically suppresses the transcription level of this operon under HC conditions. Mutation of cupAR significantly upregulated the accumulation of CupA, the key protein of NDH-1MS, under HC condition. CupAR interacted with NdhD3 and NdhF3, the membrane components of NDH-1MS, while accumulation of CupAR was reduced in the ΔndhD3 mutant. Furthermore, CupAR was colocated with CupA in both NDH-1MS complex and NDH-1S subcomplex. On the other hand, deletion of ndhR, a negative regulator of the NDH-1S operon, increased the accumulation of CupAR, whereas deletion of cupAR significantly lowered NdhR. Based on these results, we concluded that CupAR is a novel subunit of NDH-1MS, negatively regulating the activities of CupA and CO uptake dependent on NDH-1MS by positive regulation of NdhR under enriched CO conditions.