Furthermore, the employed HIF binding data in this study is for the HIF1a subunit only, whereas transcription factor cooperativity may well apply to other HIF subunits. In fact, several reports have implicated the ETS family of transcription factors in target selection by HIF2a. We observed very similar tendencies when transcriptional activation of reporter constructs was elicited by DMOG or hypoxia treatment, additionally suggesting that, at least in our experimental conditions, the contribution of these factors could occur mainly in basal conditions, as it is unlikely that hypoxia and DMOG treatment induce completely overlapping cellular responses. Several recent reports have suggested that chromatin accessibility determines HIF1 binding, although this mechanism may not fully explain HIFs binding and target selectivity. Our results indicate that an additional layer of specificity comes from proximal co-binding of other transcription factors and HIFs to open chromatin regions, thereby Benzethonium Chloride facilitating or restricting HIF-mediated transcription. Elucidation of the underlying molecular mechanisms falls outside the scope of our work, although it is tempting to speculate that transcription factors binding in proximity of HIFs may be involved in recruitment of co-activator or co-repressor proteins. Of note, a recent mammalian two-hybrid survey of protein-protein interactions for human and mouse TFs reported a physical association between HIF1A and AP-1 family member JUN, as well as the previously known interaction between CEBPB and p300. p300/CBP is a master co-activator of HIF-mediated transcription whose recruitment can also be mediated by CREB. In this regard, evidence from a synthetic transactivation screen on the EGLN1 promoter pointed to ETV4 as an additional p300-dependent coactivator of HIF-mediated transcription. Moreover, HIF1 is known to interact with Jab1/CSN5, a protein originally identified as a transcriptional coactivator for AP1. Future investigations on protein composition of HIF-bound enhancers should be pivotal in supporting this model. The associations between HIFs and AP1, CREB and CEBPs analyzed in our reporter results could be general across many HIF targets or be restricted to individual targets. To judge the generality of these results, we conducted a gene-set enrichment analysis of transcription factor targets in a sorted list of genes regulated by hypoxia. The results of this analysis showed a significant enrichment of CEBP targets among hypoxiainducible genes, suggesting that at least for this family of transcription factors, the functional association with HIFs could be relatively general. Of note, recent works have reported a direct protein-protein interaction between HIF1a and CEBPa, and have implicated CEBPa activity in regulation of the HIF target genes galectin-1 and PAI-1. Hypoxic induction of both galectin-1 and PAI-1 was found to be synergistically dependent on both HIF1a and CEBPa activity and their co-binding to the promoter region. Our results further suggest that this functional association may be general across a wider collection of HIF targets. In conclusion, the data presented herein demonstrates that integration of high-throughput chromatin immunoprecipitation and gene expression data is a successful approach to select highquality core HIF binding regions, and provides experimental proof of principle for the biological relevance of enriched transcription factor binding sites other than the HIF binding consensus in HIFmediated transcription. Specifically, our results suggest that Folinic acid calcium salt pentahydrate diverse stress-responsive transcription factors, in particular CEBPs, contribute to fine-tuning of the HIF-mediated transcriptional response. With the advent of the HIV epidemics, the disease has emerged as an important opportunistic infection in AIDS patients.