Identification of Multiple Repeat Sequences and Transcription-Related Elements Within Introns 4, 8 and 9 of Human Raf-1.

Oncogenic activation of the human c-raf-1 protein (Raf-1) occurs by truncation of its aminoterminus (regulatory domain) and retention of the carboxyterminus (kinase domain). To gain insight into the structure of human raf-1 genomic DNA corresponding to the regulatory domain of Raf-1, we analyzed the nucleotide sequences of introns 4, 8, and 9. Very high frequencies of the direct and indirect repeats, and A/T nucleotides were observed within introns 4, 8, and 9; and the DNA sequences, 5'-TTAGTCA-3' and 5'-TGATTCA-3', like the DNA recognition site of transcription factor AP-1, were identified within intron 4 of raf-1 in human normal and tumor DNAs. In addition, several transcriptional elements (CAAT box, TATA sequences, eukaryotic transcription initiation sites, and cellular transcriptional elements) were found within introns 4, 8, and 9 of human raf-1. These data suggest: 1) the high frequency of intron-specific repeats is a possible recombinational mechanism for derivation of truncated oncogenic forms of Raf-1; and 2) the multiple transcriptional elements within introns 4, 8, and 9 may have important implications in the regulation of raf-1 transcription and, therefore, its role in cell growth and proliferation.