Analysis of the cellular proto-oncogene mht/raf. Relationship to the 5′ sequences of v-mht in avian carcinoma virus MH2 and v-raf in murine sarcoma virus 3611

Abstract The avian carcinoma virus MH2 contains a hybrid gene Δ gag-mht with a contiguous open reading frame of 2682 base pairs as well as v- myc and avian helper virus-related sequences. Δ gag is a partial retroviral core protein gene while v- mht and v- myc are cell-drived sequences. The v- mht sequence can be divided into two regions: the v- raf -related region at its 3′ end contains 969 nucleotides which are 94% related as amino acid sequence to the onc -specific v- raf sequence of murine sarcoma virus 3611 (MSV 3611), and the v- mht -specific region at its 5′ end contains 173 nucleotides which are unrelated to either MSV 3611 or avian helper virus sequences. To study the origin of the v- mht -specific sequences, the 5′ region of the proto- mht/raf gene was molecularly cloned from a phage lambda library containing genomic chicken sequences. Nucleic acid hybridization, heteroduplex and DNA sequence analyses indicate that the v- mht -specific sequences are encoded in three exons. The first and second exons are separated by a 3.4-kb intron while the second and third exons are separated by a 90-bp intron. The last 14 by of the third exon are shared with v- raf and thus represent the start of v- raf -related sequences. The junction between v- mht -unrelated and related cellular sequences occurs within the first exon. There is no homology between the v- mht -unrelated sequences and the retroviral helper sequences indicating that the viral transduction of the proto- mht/raf sequences occurred through illegitimate recombination. The predominant v- mht -related messenger RNA (4.0 kb) hybridizes to several noncontiguous regions on the molecularly cloned cellular proto- mht/raf DNA indicating that the proto- mht/raf gene is distributed over at least 10 kb of DNA in the chicken genome. Thus the v- mht oncogene is a subset of its normal cellular homolog in that it lacks intervening sequences and possibly lacks 5′-coding sequences.