not really finished is crucial for development of the zebrafish outer retina and encodes a transcription factor highly homologous to human Nuclear Respiratory Factor-1 and avian Initiation Binding Repressor

Not really finished (nrf), a larval-lethal mutation in zebrafish generated by retroviral insertion, causes specific retinal defects, Analysis of mutan t retinae reveals an extensive loss of photoreceptors and their precursors around the onset of visual function, These neurons undergo apoptosis during differentiation, affecting all classes of photoreceptors, suggesting an es sential function of nrf for the development of all types of photoreceptors, In the mutant, some photoreceptors escape cell death, are functional and, as judged by opsin expression, belong to at least three classes of cones an d one class of rods. The protein encoded by nrf is a close homologue of hum an Nuclear Respiratory Factor 1 and avian Initiation Binding Repressor, tra nscriptional regulators binding the upstream consensus sequence RCGCRYGCGY, At 24 hours of development, prior to neuronal differentiation, nrf is expr essed ubiquitously throughout the developing retina and central nervous sys tem, At 48 hours of development, expression of nrf is detected in the gangl ion cell layer, in the neurons of the inner nuclear layer, and in the optic nerve and optic tracts, and, at 72 hours of development, is no longer dete ctable by in situ hybridization, Mutants contain no detectable nrf mRNA and die within 2 weeks postfertilization as larvae with reduced brain size. On the basis of its similarity with NRF-1 and IBR, nrf is likely involved in transcriptional regulation of multiple target genes, including those that e ncode mitochondrial proteins, growth factor receptors and other transcripti on factors. This demonstrates the power of insertional mutagenesis as a mea ns for characterizing novel genes necessary for vertebrate retinal developm ent.