ORGAN-SPECIFIC DIFFERENTIAL REGULATION OF A PROMOTER SUBFAMILY FOR THE RIBULOSE-1,5-BIPHOSPHATE CARBOXYLASE OXYGENASE SMALL-SUBUNIT GENES IN TOMATO

The tomato (Lycopersicon esculentum) gene family for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (RBCS) has been in vestigated to determine the role of promoter regions and DNA-protein i nteractions in the differential organ-specific transcription of indivi dual genes. Transgenic plants expressing RBCS-promoter-beta-glucuronid ase fusion genes have confirmed that promoter fragments ranging from 0 .6 to 3.0 kb of the RBCS1, RBCS2, and RBCS3A genes were sufficient to confer the temporal, organ-specific, and differential expression patte rn observed for the endogenous genes. The individual temporal and orga n-specific beta-glucuronidase enzyme activities closely reflect the qu alitative and quantitative transcription activities of the respective RBCS genes, including the strongly reduced activity of RBCS3A (L.A. Wa nner, W. Gruissem [1991] Plant Cell 3: 1289-1303). In particular, tiss ue-specific activity of all three promoters is similar in developing f ruit, with high activity in the locular tissue and extremely reduced a ctivity in the pericarp. This specific pattern of gene activity was fu rther substantiated by in situ analysis of RBCS mRNA levels. Together, the data suggest an interesting correlation between RBCS gene activit y and sink strength in different fruit tissues. DNA-protein interactio n studies have revealed a novel fruit-specific DNA-binding protein cal led FBF that specifically interacts with a sequence element directly u pstream of the G-box in the RBCS3A promoter. FBF binding thus correlat es with the reduced activity of this promoter in developing tomato fru it, rendering it a candidate for a fruit-specific negative regulator o f transcription in tomato.