NUCLEAR RESPIRATORY FACTOR-1 AND FACTOR-2 UTILIZE SIMILAR GLUTAMINE-CONTAINING CLUSTERS OF HYDROPHOBIC RESIDUES TO ACTIVATE TRANSCRIPTION

Nuclear respiratory factors 1 and 2 (NRF-1 and NRF-2) are ubiquitous t ranscription factors that have been implicated in the control of nucle ar genes required for respiration, heme biosynthesis, and mitochondria l DNA transcription and replication. Recently, both factors hare been found to be major transcriptional determinants for a subset of these g enes that define a class of simple promoters involved in respiratory c hain expression, Here, functional domains required for transactivation by NRF-1 have been defined, An atypical nuclear localization signal r esides in a conserved amino-terminal region adjacent to the DNA bindin g domain and consists of functionally redundant clusters of basic resi dues. A second domain in the carboxy-terminal half of the molecule is necessary for transcriptional activation, The activation domains of bo th NRF-1 and NRF-2 were extensively characterized by both deletion and alanine substitution mutagenesis, The results show that these domains do not fall into known classes defined by a preponderance of amino ac id residues, including glutamines, prolines, or isoleucines, as found in other eukaryotic activators, Rather, in both factors, a series of t andemly arranged clusters of hydrophobic amino acids were required for activation. Although all of the functional clusters contain glutamine s, the glutamines differ from the hydrophobic residues in that they ar e inconsequential for activation, Unlike the NRF-2 domain, which conta ins its essential hydrophobic motifs within 40 residues, the NRF-1 dom ain spans about 40% of the molecule and appears to have a bipartite st ructure, The findings indicate that NRF-1 and NRF-2 utilize similar hy drophobic structural motifs for activating transcription.