The mammalian genome contains several hundred thousand sequence motifs (GAS sites) that can represent binding motifs for the transcription factor STAT5 (GAS sites). Although many STAT5 target genes have been identified, it is not known what percentage of genes with GAS motifs bind to and are regulated by STAT5. Towards this end, we have investigated genome-wide binding of STAT5 in mouse embryonic fibroblasts upon cytokine stimulation and monitored the ensuing gene activation. More than 20,000 genomic sites were bound by STAT5, however, the majority of nearby genes was not under cytokine-STAT5 control. To further address whether binding of transcription factors is modulated by its concentration we overexpressed STAT5 20-fold. Although additional genomic areas of STAT5 occupancy were detected, there was little consequence on the expression of associated genes. This study, for the first time, demonstrated the magnitude of opportunistic genomic STAT5 binding that does not result into transcriptional activation of neighboring genes. It can be anticipated that similar scenarios exist for other transcription factors. We are now in search of a biological significance of this opportunistic transcription factor binding to gain insight into aberrant gene regulation in malignancies with activating Stat5 mutations.
