Expression and properties of the mitochondrial and cytosolic forms of fumarase in sunflower cotyledons.

Fumarase (EC 4.2.1.2) is encoded in sunflower (Helianthus annuus L.) by two genes (FUM1 and FUM2) expressing correspondingly the mitochondrial and the cytosolic form. Both forms have been purified from sunflower cotyledons and characterized. Three quarters of fumarase activity is located in the mitochondrial and one quarter in the cytosolic fraction. The cytosolic form has lower pH optimum than the mitochondrial form, it possesses higher affinity to malate, activated by Mn2+ and less efficiently by Mg2+ while the mitochondrial form is activated only by Mg2+. It is proposed that the mitochondrial form is involved in the respiratory processes linked to the tricarboxylic acid cycle and the cytosolic form participates in the utilization of succinate produced in the glyoxylate cycle providing the flux to gluconeogenesis in germinating sunflower seeds.

Light inhibition of fumarase in Arabidopsis leaves is phytochrome A-dependent and mediated by calcium.

Inhibition of fumarase activity in the light has been studied in Arabidopsis in relation to the involvement of phytochrome. Using knockout phytochrome mutants, we observed that the main regulator of FUM1 gene transcription, encoding the mitochondrial form of fumarase, is phytochrome A. The active form of phytochrome A suppressed FUM1 expression, while the expression of the FUM2 gene encoding the cytosolic form of fumarase was unaffected both in darkness and in light. The nuclear concentration of Ca(2+) was modulated by red and far-red light. We suggest that the signal transduction mechanism operates via Ca(2+) activation of expression of the gene encoding the transcription factor PIF3, which binds to promoters of phytochrome-regulated genes and inhibits FUM1 expression.