Role for RFX transcription factors in non-neuronal cell-specific inactivation of the microtubule-associated protein MAP1A promoter

Microtubule-associated protein MAP1A is expressed abundantly in mature neurons and is necessary for maintenance of neuronal morphology and localization of some molecules in association with the microtubule-based cytoskeleton. Previous studies indicated that its complementary expression together with MAP1B during nervous system development is regulated at the transcriptional level and that the mouse Map1A gene is transcribed under the control of 5′ and intronic promoters. In this study, we investigated the regulatory mechanisms that govern the neuronal cell-specific activation of the MAP1A 5′ promoter. We found that two regulatory factor for X box (RFX) binding sites in exon1 of both the mouse and human genes are important for effective transcriptional repression observed only in non-neuronal cells by reporter assays. Among RFX transcription factor family members, RFX1 and 3 mainly interact with repressive elements in vitro. Cotransfection studies indicated that RFX1, which is expressed ubiquitously, down-regulated the MAP1A 5′ promoter activity in nonneuronal cells. Unexpectedly, RFX3, which is abundantly expressed in neuronal cells, down-regulated the transactivity as well, when it was expressed in nonneuronal cells. Both RFX1 and 3 did not down-regulate the transactivity in neuronal cells. These results suggest that RFX1 and 3 are pivotal factors in down-regulation of the MAP1A 5′ promoter in non-neuronal cells. The cell type-specific down-regulation, however, does not depend simply on which RFX interacts with the elements, but seems to depend on underlying profound mechanisms.