2 INDEPENDENT NUCLEAR-LOCALIZATION SIGNALS ARE PRESENT IN THE DNA-BINDING HIGH-MOBILITY GROUP DOMAINS OF SRY AND SOX9

SRY and SOX9, members of the family of high-mobility group (HMG) domai n transcription factors, are both essential for testis formation durin g human embryonic development. The HMG domain is a DNA-binding and DNA -bending motif comprising about 80 amino acid residues. It has been sh own that SRY and SOX9 are nuclear proteins. Using normal or mutant SRY -beta-galactosidase and SOX9-beta-galactosidase fusion proteins in tra nsfection studies involving COS-7 cells, we have identified two nuclea r localization signals (NLSs) within the HMG domains of both proteins that can independently direct the fusion proteins into the nucleus. On ly mutational inactivation of both NLS motifs resulted in complete exc lusion of the fusion proteins from the nucleus. The NLS sequences are located at the N and C termini of the HMG domain and are a bipartite N LS motif and a basic cluster NLS motif, respectively. Both NLS motifs are conserved in the HMG domains of other transcription factors. The i mplications of the present results are discussed regarding (a) the app arent dual function of certain basic amino acid residues in the HMG do main of SRY in both DNA binding and in nuclear localization and (b) th e possible control of SOX9 in early gonadal differentiation at the lev el of nuclear translocation.