Psu, a coat protein from bacteriophage P4, inhibits Rho-dependent transcription termination both in vivo and in vitro. The Psu protein is alpha-helical in nature and appeared to be a dimer in solution. It interacts with Rho and affects the ATP binding and RNA-dependent ATPase activity of Rho, which in turn reduces the rate of RNA release from the elongation complex. Crystals of Psu were grown in space group I422 in the presence of PEG, with unit-cell parameters a = b = 148.76, c = 63.38 A and a calculated Matthews coefficient of 2.1 A(3) Da(-1) (41.5% solvent content), assuming the presence of two molecules in the asymmetric unit. A native data set was collected to 2.3 A resolution.