Regulation of gene expression involves the orchestrated interaction of a large number of proteins with transcriptional regulatory elements in the context of chromatin. Our understanding of gene regulation is limited by the lack of a protein measurement technology that can systematically detect and quantify the ensemble of proteins associated with the transcriptional regulatory elements of specific genes. Here, we introduce a set of selected reaction monitoring (SRM) assays for the systematic measurement of 464 proteins with known or suspected roles in transcriptional regulation at RNA polymerase II transcribed promoters in Saccharomyces cerevisiae. Measurement of these proteins in nuclear extracts by SRM permitted the reproducible quantification of 42% of the proteins over a wide range of abundances. By deploying the assay to systematically identify DNA binding transcriptional regulators that interact with the environmentally regulated FLO11 promoter in cell extracts, we identified 15 regulators that bound specifically to distinct regions along ∼600 bp of the regulatory sequence. Importantly, the dataset includes a number of regulators that have been shown to either control FLO11 expression or localize to these regulatory regions in vivo. We further validated the utility of the approach by demonstrating that two of the SRM-identified factors, Mot3 and Azf1, are required for proper FLO11 expression. These results demonstrate the utility of SRM-based targeted proteomics to guide the identification of gene-specific transcriptional regulators. C ritical to understanding gene regulation is the ability to determine the composition of the regulatory complexes that assemble at specific genes and to determine how the composition of these complexes change in response to cellular, genetic, and environmental signals. Despite considerable efforts to address these key questions, the lack of methods for routine analysis of the ensemble of transcription factors (TFs) associated with specific transcriptional regulatory elements (TREs) remains a significant limitation.Current approaches for studying TF-TRE interactions include the EMSA (1, 2), protein binding microarrays (PBMs) (3), the yeast one-hybrid method (4), and chromatin immunoprecipitation (ChIP)-based methods (5, 6). Although each of these methods can provide information about TF-DNA interactions, their utility for routine analysis of TF-DNA interactions and complexes assembled at TREs is limited by the need for genetic engineering, protein detection reagents, and/or purified proteins. Notably, ChIP-chip has been used to systematically study the localization of most transcriptional regulators (TRs) across the yeast genome (7), but this tour de force required the creation and independent assay of 203 TR-specific, epitope-tagged strains.Another approach for studying TF-TRE interactions that does not require genetic engineering or protein detection reagents, and can readily provide information about the ensemble of TFs associated with a TRE, is DNA affinity purification fo...