DELETIONS WITHIN THE TRANSFORMATION-SPECIFIC RNA SEQUENCES OF ACUTE-LEUKEMIA VIRUS-MC29 GIVE RISE TO PARTIALLY TRANSFORMATION-DEFECTIVE MUTANTS

The viral RNA of 3 nonconditional mutants of avian myelocytomatosis virus MC29 were analyzed. These mutants, which were originally isolated from the quail producer line Q10 and were designated 10A, 10C and 10H, have lost most of the ability to transform hematopoietic cells in vitro and to induce tumors in vivo but still transform cultured [quail] fibroblasts with the same efficiency as wild-type (wt) MC29. Electrophoretic analyses showed that the mutant genomic RNA were smaller than the 5.7-kilobase genome of wt MC29; the genomes of mutants 10A, 10C and 10H were .apprx. 5.5, 5.3 and 5.1 kilobases long, respectively. Analyses of the transformation-specific sequences of these mutant RNA by a combination of T1 oligonucleotide fingerprinting and hybridization with c[complementary]DNA from the transformation-specific sequences myc of wt MC29, or competition hybridization including wt MC29 RNA, revealed that deletions of myc-sequences had occurred. The deletions in all 3 mutants overlapped, since all had lost 1 particular myc-specific oligonucleotide. In agreement with the size of the genomic RNA, mutants 10C and 10H had lost 2 additional myc oligonucleotides and mutant 10A contained a modified myc oligonucleotide. The locations of the deletions were deduced from comparisons with previously established oligonucleotide maps of several members of the MC29 subgroup of actue leukemia viruses and by hybridization of wt and mutant RNA to molecularly cloned subgenomic fragments of wt MC29 proviral DNA; this represents the 5'' and 3'' domains of the myc sequence. The deleted sequences represented overlapping internal segments of the myc sequence and the borders of myc with the partial complements of the virion genes gag and env appeared to be conserved in mutant and wt MC29 RNA. The correlation between the altered transforming potential for hematopoietic cells and the partial deletion of myc in the mutant RNA provided direct genetic evidence for the involvement of myc in oncogenesis. The unaffected efficiency of these mutants in fibroblast transformation suggested that the deleted sequences are not essential for the fibroblast-transforming potential of the onc gene of MC29.