Nuclear respiratory factor (NRF)-1 is a transcription factor with a novel function in neurite outgrowth. Synapsin I protein is a well-known phosphoprotein in neuronal terminals and has been implicated in neuronal differentiation. Human synapsin I gene promoter has a putative NRF-1 responsive element (NRE), but it is not known whether this NRE is functional. We hypothesized that synapsin I is downstream of NRF-1 and mediates its function in neurite outgrowth. Gel electrophoretic mobility shift assays, chromatin immunoprecipitation, site-directed mutagenesis, and promoter studies indicated that NRF-1 is a positive regulator of synapsin I promoter. Exogenous NRF-1 overexpression increased synapsin I protein levels in IMR-32 and HEK293T cells. Serum deprivation, which induces neurite outgrowth in IMR-32 cells, increased the binding activity of NRF-1 to synapsin I NRE and induced alternating synapsin I protein expression. Down-regulating synapsin I expression markedly decreased the percentage of neurite-bearing cells and the length of the longest neurite in IMR-32 cells that stably or transiently overexpressed NRF-1. We conclude that the human synapsin I gene is positively regulated by NRF-1 and mediates the function of NRF-1 in neurite outgrowth. © 2009 Wiley-Liss, Inc.