Heterotrimeric G proteins are an important component of signal transduction pathway in animals. Although these proteins have been described in plants, their exact function and action mode are not clearly defined. In order to analyze the relationship between these proteins and the transduction of light signals in spinach, we have isolated by 5' and 3' RACE-PCR a 1660bp cDNA clone called SOGA1. This codes for a 383aa protein, which reveals a very strong homology with other plant Galpha subunit sequences. Genomic analysis suggested that SOGA1 belonged to a small multiple gene family. Northern blots and in-situ hybridization analyses showed that SOGA1 transcripts accumulate in all organs tested with a specific high level associated with the apex, roots and hypocotyls. Finally, a time-course analysis performed on the green tissues showed that accumulation of SOGA1 transcripts follows a circadian rhythm. However, in-situ hybridization analysis of the apex suggested the opposite behavior, while no variation was observed in the hypocotyl.