A cDNA clone encoding mitochondrial citrate synthase (EC 4.1.3.7), the first enzyme of the tricarboxylic-acid cycle, was isolated from potato (Solanum tuberosum L.) and expression of the enzyme analyzed. The deduced amino-acid sequence of the potato mitochondrial citrate synthase showed high similarity to known citrate synthases from fungi, mammals and Arabidopsis thaliana. The expression pattern of this clone was determined by Northern blot analysis. Expression was detected in all tissues analyzed. The highest level of expression was found in green flower buds. In photosynthetic tissues, stronger mRNA expression was detected in mature than in immature leaves. This rise in expression with leaf age was accompanied by an increase in citrate-synthase activity. Within flowers, expression was severalfold stronger in anthers than in ovaries, indicating a role of mitochondrial citrate synthase during anther or pollen development. A comparatively low level of transcript was detected in underground heterotrophic tissues, such as stolons, tubers and roots. When tubers were stored at low temperature (4 degrees C), mitochondrial citrate-synthase gene expression increased slightly. From the data obtained, we conclude that expression of the mitochondrial citrate-synthase gene is regulated by developmental and environmental factors. The relatively high expression in leaves is in line with the assumption that mitochondria play an important role in photosynthetically active tissues.