ACTIVATION OF GENE-TRANSCRIPTION BY THE AMINO-TERMINUS OF THE N-MYC PROTEIN DOES NOT REQUIRE ASSOCIATION WITH THE PROTEIN ENCODED BY THE RETINOBLASTOMA SUPPRESSOR GENE RB1

N-Myc encodes a nuclear phosphoprotein that contains a basic region (B R), a helix-loop-helix (HLH) and a leucine zipper (Zip). These motifs are hallmarks of certain transcription factors. In pursuit of the ques tion if N-Myc can activate transcription, we have employed an experime ntal model involving the yeast transcription factor Gal4. We have firs t generated fusion proteins containing the Gal4 DNA-binding domain joi ned to portions of N-Myc. Subsequently we have analysed if chimeric pr oteins can transactivate the transcription of a reporter under the con trol of Gal4 binding sites. Here we show that the amino terminal porti on of N-Myc activates transcription. Activation maps to a domain highl y conserved among Myc-proteins and to other non-conserved sequences, s uggesting functional redundancy. Previous studies had documented in vi tro association of the RBI protein with N-Myc (Rustig et al., 1991). W e here confirm this observation and identify the region encompassing t he transactivation domain as responsible for RBI binding. Analyses of N-Myc transactivation in retinoblastoma cell line WERI lacking a funct ion RBI protein gave results similar to those with cell lines having a n intact RBI protein, showing that RBI protein is not required for tra nsactivation by N-Myc. The present findings leave open the question if deregulated expression of N-Myc contributes to tumorigenesis by trans criptional activation of as yet unidentified target genes or by functi onally inactivating the protein encoded by the tumor suppressor gene R B1, or by a combination of both.