Mutants of the cyanobacterium, Synechococcus PCC7942, lacking active CO2 uptake uncover highly specialised NADPH dehydrogenase complexes involved in CO2 uptake

Abstract

Cyanobacteria grown under inorganic carbon (Ci) limitation possess at least 4 modes of active uptake for Ci (CO2 and HCO3-) each showing a high degree of functional redundancy. These modes of uptake are an integral part of the cyanobacterial CO2 concentrating mechanism (CCM) which functions to saturate CO2 levels around Rubisco. We have employed a new approach to obtain mutants with defects in the CCM that involves further tagging mutagenesis of an existing Synechococcus PCC7942 mutant that has defects in two types of high affinity Ci uptake, namely cmpABCD (inducible, high affinity HCO3- uptake) and ndhD3 (inducible, high affinity CO2 uptake). Using this approach we were able to identify several new CCM mutants. We isolated ndhD4, coding for a subunit of a specialised NADPH dehydrogenase (NDH-I) complex, and confirmed that like the situation found for Synechocystis PCC6803 (Ohkawa et al., 2000, J Biol. Chem. 275; 31630-4) a double mutant in ndhD3 and ndhD4 produced a complete loss of CO2 uptake activity. This and other evidence indicates that Synechococcus PCC7942 possesses several types of NDH-I complexes with different functions. The complex possessing NdhD3, NdhF3 Orf430 subunits, NDH-I3, is involved in inducible, high affinity CO2 uptake, whereas another complex possessing NdhF4 and NdhD4 subunits, NDH-I4, is involved in constitutive, low affinity CO2 uptake. Particular emphasis has been applied to understand the role of Orf430 and to attempt to describe the mechanism of involvement of NDH-I3/4 complexes in CO2 uptake. A new mechanistic model for CO2 uptake will be presented.