Positive and negative determinants of target gene specificity in myb transcription factors.

The A-Myb and c-Myb transcription factors share a highly conserved DNA-binding domain and activate the same promoters in reporter gene assays. However, the two proteins have distinct biological activities, and expressing them individually in human cells leads to the activation of distinct sets of endogenous genes, suggesting that each protein has a unique transcriptional specificity. Here, the structural and functional features of the Myb proteins were compared, using assays of endogenous gene expression to measure changes in specificity. When the Myb proteins were tested in different cell types, they activated unique and nearly nonoverlapping sets of genes in each cellular context. Deletion and domain swap experiments identified small, discreet positive and negative elements in A-Myb and c-Myb that were required for the regulation of specific genes, such as DHRS2, DSIPI, and mim-1. The results suggest that individual functional elements in the transcriptional activation domains are responsible for activating specific cellular genes in a context-specific manner. The results also have important implications for interpreting results from reporter gene assays, which fail to detect the differences in activity identified through endogenous gene assays, and fusion protein constructs that alter the transcriptional activation domains and the activities of the Myb proteins.

Distinct changes in gene expression induced by A-Myb, B-Myb and c-Myb proteins.

The c-Myb, A-Myb and B-Myb transcription factors have nearly identical DNA-binding domains, activate the same reporter gene constructs in animal cells, but have different biological roles. The Myb proteins are often coexpressed in the same cells, raising questions about whether they activate similar or distinct gene expression profiles, and whether they cooperate or compete in regulating the same promoters. Here, recombinant adenoviruses were used to express each protein in human mammary cells, and then microarray assays were used to assess global changes in gene expression. Each Myb protein induced a unique and specific set of changes, displaying activities far more complex than revealed by standard reporter gene assays. These results have important implications for the roles of various Myb proteins in normal and transformed human cells, for regulatory pathways that might modify their activities and for the importance of acquired mutations that may qualitatively alter their functions in tumors.