Does Rubisco Limit the Rate of Photosynthesis?

Abstract

Rubisco is considered the main ratelimiting factor of photosynthesis under saturating irradiance and limiting CO2 concentrations. We examined the extent of limitation imposed by Rubisco on photosynthesis of the amphibious plant Nuphar lutea and the cyanobacterium Synechocystis PCC6803. In Nuphar aerial leaves we found a daily rhythm in the light-saturated rate of photosynthesis resulting from synchronous alterations in stomatal opening and Rubisco activity. In Synechocystis we found that point mutations (including three at the catalytic site), which reduced the catalytic turnover of Rubisco (Kcat) up to 10% of its wild type value and increased the Km(RuBP), had little to moderate effect on the rate of photosynthesis and hardly affected the photoautotrophic growth. In some mutants the inferior kinetic parameters of the enzyme were compensated by massive increase in its content. By correlating the photosynthesis rate at saturating substrate concentrations (Pmax) with Rubisco activity/cell in various Rubisco mutants, we found that unlike in higher plants, the cyanobacterial photosynthesis is hardly limited by Rubisco activity. As a result, a decrease of up to 2/3 in Rubisco activity/cell has no effect on its Pmax.