The v‐Myb DNA‐binding domain differs from that of c‐Myb mainly by deletion of the first of three repeats. This truncation correlates with efficient oncogenic transformation and a decrease in DNA‐binding activity. Here we demonstrate that the D‐type cyclins, cyclin D1 and D2 in particular, specifically inhibit transcription when activated through the v‐Myb DNA‐binding domain, but not the c‐Myb DNA‐binding domain. Analysis of a cyclin D1 mutant and a dominant‐negative CDK4 mutant implied that this repression is independent of complex formation with a CDK partner. Association of cyclin D1 and D2 with the Myb DNA‐binding domain could be demonstrated. Increased levels of cyclin D1 and D2 resulted in a stabilization of the Myb proteins, but not in an alteration in binding of the Myb proteins to DNA. These results highlight an unexpected role for cyclin D as a CDK‐independent repressor of transcriptional activation by v‐Myb but not c‐Myb. This differential effect of D‐type cyclins on v‐Myb and c‐Myb might help to explain the mechanism underlying the oncogenic activity of v‐Myb, which appears to be a stronger transcriptional activator following the TPA‐induced differentiation of transformed monoblasts when cyclin D1 and D2 are down‐regulated.

Both viral Myb (v-Myb) and cellular Myb (c-Myb) are nuclear sequence-specific DNA-binding proteins that can function as transcriptional activators. v-Myb, encoded by avian myeloblastosis virus, induces acute monoblastic leukemia in chickens and transforms avian myelomonocytic cells in culture. The normal c-Myb protein is essential for hematopoietic development. Previous reports suggested that truncation of c-Myb is required for oncogenic transformation of avian myelomonocytic cells in culture. In this study, we demonstrate that constitutive expression of full-length c-Myb can transform avian myelomonocytic cells isolated from embryonic yolk sacs by using a strategy to enhance the efficiency of infection and/or expression of c-myb-containing viruses. c-Myb-transformed myelomonocytic cells display a different phenotype than cells transformed by v-MybAMV or other Myb mutants. c-Myb-transformed yolk sac cells are heterogeneous populations with characteristics of both the macrophage and granulocyte lineages. Our results demonstrate that constitutive expression of full-length c-Myb is sufficient to activate its oncogenic potential, but that the target cells for c-Myb are relatively rare and presumably quite immature.

The nuclear protein v-Myb, encoded by the avian myeloblastosis virus (AMV), can induce acute monoblastic leukemia in vivo and transform chicken myelomonocytic cells in culture, The N terminus of v-Myb functions as the DNA-binding domain, and multiple central and C-terminal regions of this protein have been reported to function in transcriptional activation of model reporter genes, We showed previously that a C-terminal domain (amino acids 296 to 371) is required for transcriptional activation and transformation of primary chicken myelomonocytic cells, In this study, we have now analyzed a series of C-terminal mutants of v-Myb to further investigate this domain, A strong correlation was observed between transcriptional activation and leukemic transformation by this series of mutants. Furthermore, deletion analyses demonstrate that the C-terminal 41 amino acids of v-Myb(AMV) (amino acids 331 to 371 of the Myb portion) are nonessential whereas further deletion of amino acids 321 to 330 (EFAETLQLID) results in a nonfunctional protein, Hence, we defined a 10-amino-acid subregion (the "FAETL" motif) required for transcriptional activation and oncogenic transformation by v-Myb(AMV). The FAETL region is part of a putative leucine zipper structure and lies near a cluster of phosphorylation sites. Our analysis of mutants with substitutions of the zipper leucines or multiple adjacent phosphorylation sites demonstrates that the function of the FAETL motif is not dependent on an intact leucine zipper structure or adjacent phosphorylation sites, The study of GAL4-Myb fusions suggests that this region is important in maintaining a fully functional conformation of v-Myb, The putative leucine zipper structure has previously been proposed to exert inhibitory effects on c-Myb because its mutation caused increased transcriptional transactivation and transformation, Interestingly, our results show that this region is essential for the functions of v-Myb without requiring a heptad leucine repeat.