We are far from understanding how complex transcriptional programs are generated from genomic information. In October 25 Science, Tong Lee and colleagues report how they constructed a map of transcriptional regulatory networks in Saccharomyces cerevisiae by identifying all the sites bound by a large number of transcriptional regulators (Science, 298:799-804, October 25, 2002).

Lee et al. performed genome-wide location analysis to identify binding sites for all 141 transcription factors listed in the Yeast Proteome Database. They introduced a myc epitope tag into the genomic sequences encoding each factor. Using immunoprecipitation and microarray analysis almost 4,000 regulator-DNA interactions could be defined.

More than a third of all yeast genes were bound by one or more of the regulators and many were bound by multiple factors. The average number of gene promoters bound by an individual factor was 38. These data were used to construct regulatory networks that are...

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