The nucleotide sequences of the complementary DNA of pyruvate, Pi dikinase (PPDK) from Flaveria bidentis, a C4 plant which possesses a cold-sensitive form of PPDK, and Flaveria brownii, a 'C4-like' plant which possesses a cold-tolerant form of PPDK, were determined. PPDK was isolated from the leaves of both Flaveria species and purified and the N-terminal amino acid sequences characterised. Together with a maize PPDK cDNA, cDNA inserts which code for the mature form of PPDK of F. bidentis and of F. brownii were expressed in bacteria and the cold sensitivity of the expressed PPDK studied. The cold sensitivity of the PPDK expressed in bacteria mimics the cold sensitivity of PPDK found in vivo in all three plant species. This study indicates that the cold sensitivity of plant PPDK is controlled by the primary structure of the enzyme.