Bacteriophage T4 encodes a transcription factor, MotA, that binds to the -30 region of middle-mode promoters and activates transcription by host RNA polymerase. We have solved the structure of the MotA activation domain to 2.2 A by X-ray crystallography, and have also determined its secondary structure by NMR. An area on the surface of the protein has a distinctive patch that is populated with acidic and hydrophobic residues. Mutations within this patch cause a defective T4 growth phenotype, arguing that the patch is important for MotA function. One of the mutant MotA activation domains was purified and analyzed by NMR, and the spectra clearly show that the domain is properly folded. The mutant full-length protein appears to bind DNA normally but is deficient in transcriptional activation. We conclude that the acidic/hydrophobic surface patch is specifically involved in transcriptional activation, which is reminiscent of eukaryotic acidic activation domains.