Suspension cells of tobacco (Nicotiana tabacum 1. cv Bright Yellow) were used to investigate signals regulating the expression of the nuclear gene Aoxl encoding the mitochondrial alternative oxidase (AOX) protein responsible for cyanide-resistant respiration in plants. We found that an increase i n the tricarboxylic acid cycle intermediate citrate (either after its exogenous supply to cells or after inhibition of aconitase by monofluoroacetate) caused a rapid and dramatic increase i n the steady-state level of

The interaction of PsaC with the core heterodimer of Photosystem 1 was studied in wild type Synechocystis sp. PCC 6803 and the site-directed mutant R561E of PsaB. The mutant reaction center was much less stable in urea and the functional reconstitution of the mutant core using PsaC was impaired. However, the extent of reconstitution increased in the presence of divalent cations whereas that of the wild type was inhibited. We conclude that the reaction center in the mutant is unstable, most likely due to the introduction of an unfavorable electrostatic interaction between surface-exposed residues on PsaC and the binding site for the subunit on the core heterodimer in support of the model proposed previously (Rodday et al. (1993) Photosynth Res 36: 1-9).

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