Gene structure and differential expression of acidic fibroblast growth factor mRNA: identification and distribution of four different transcripts.

We have isolated four cDNA clones coding for human acidic fibroblast growth factor (aFGF) containing alternative 5' untranslated exons. Using RNAase protection analyses, we demonstrated the presence of at least four upstream, untranslated exons that are alternatively spliced to the first protein-coding exon. We designate these four untranslated exons, -1A, -1B, -1C and -1D. Splicing of these exons to the first coding exon will generate mRNA 1.A, 1.B, 1.C and 1.D respectively. Expression of these transcripts is regulated in a tissue-specific manner, as the major aFGF transcript in human brain frontal cortex differs from that in kidney. Furthermore, the pattern of aFGF transcripts in several glioblastoma cell lines tested is different from that in normal brain tissue. We isolated nine overlapping genomic clones containing these four upstream, untranslated exons. These four exons were localized on these clones by Southern hybridization and nucleotide sequence analysis. The overlapping clones are shown to be contiguous with our previously isolated genomic clones that contain the three aFGF-coding exons. The sizes of the four introns are 82.9, 71.1, 29.3 and 6.9 kbp. The transcriptional start sites of the two most upstream exons (-1A and -1B) have been mapped using RNAase protection and primer extension analyses. The sequences upstream of the start sites for aFGF 1.B mRNA do not contain a consensus TATA box. In contrast, the canonical CCAAT and TATA sequences are located at the proper distances from the transcription start site of aFGF 1.A mRNA.