CLONING AND CHARACTERIZATION OF AN ATBF1 ISOFORM THAT EXPRESSES IN A NEURONAL DIFFERENTIATION-DEPENDENT MANNER

The human ATBF1 cDNA reported previously, now termed ATBF1-B, encodes a 306-kDa protein containing 4 homeodomains and 18 zinc fingers includ ing one pseudo zinc finger motif, Here, we report the isolation of a s econd ATBF1 cDNA, 12 kilobase pairs long, termed ATBF1-A. The deduced ATBF1-A protein is 404 kDa in size and differs from ATBF1-B by a 920-a mino acid extention at the N terminus, Analysis of 5'-genomic sequence s showed that the 5'-noneoding sequences specific to ATBF1-A and ATBF1 -B transcripts were contained in distinct exons that could splice to a downstream exon common to the ATBF1-A and ATBF1-B mRNAs. The expressi on of ATBF1-A transcripts increased to high levels when P19 and NT2/D1 cells were treated with retinoic acid to induce neuronal. differentia tion, Preferential expression of ATBF1-A transcripts was also observed in developing mouse brain. Transient transfection assays showed that the 5,5-kilobase pair sequence upstream of the ATBF1-A-specific exon ( exon 2) supported expression of the linked chloramphenicol acetyltrans ferase gene in neuronal cells derived from P19 cells but not in undiff erentiated P19 or in F9 cells, which do not differentiate into neurons . These results showed that ATBF1-A and ATBF1-B transcripts are genera ted by alternative promoter usage combined with alternative splicing a nd that the ATBF1-A-specific promoter is activated during neuronal dif ferentiation.