The enzyme ribulose diphosphate carboxylase (RudPCase) has been shown to be present in high concentrations in chloroplasts (Lyttleton and Tso, 1958) however, the measured turnover of the isolated enzyme is so low that the exact role of this enzyme in CO2 fixation has been questioned (Trown, 1965; Gibbs et al., 1967). In addition the levels of CO2 required for efficient functioning of purified RudPCase are so high relative to the concentration of CO2 in the atmosphere that calculations by various workers have concluded that the enzyme could not support the known CO2 fixation rate in intact tissue. As a result, extensive investigations have been carried out to determine means of preserving or restoring activity to the isolated enzyme. This communication describes the isolation of a small factor both from tomato leaves and from isolated chloroplasts which increases the enzyme activity of ribulose diphosphate carboxylase both in crude extracts and in the isolated enzyme preparations. The factor has a chromophoric group with absorption maxiumum at 325 mμ. Moreover, the activation of RudPCase is light dependent with a maximum in the action spectrum also a 325 mμ. © 1969.
