TRANSCRIPTION elongation factors stimulate the activity of DNA-dependent RNA polymerases by increasing the overall elongation rate and the completion of RNA chains. One group of such factors, which includes Escherichia coli GreA, GreB and eukaryotic SII (TFIIS), acts by inducing hydrolytic cleavage of the transcript within the RNA polymerase, followed by relase of the 3’-terminal fragment(1-5). Here we report the crystal structure of GreA at 2.2 Angstrom resolution. The structure contains an amino-terminal domain consisting of an antiparallel alpha-helical coiled-coil dimer which extends into solution, reminiscent of the coiled coil in seryl-tRNA synthetases(6). A site near the tip of the coiled-coil ’finger’ plays a direct role in the transcript cleavage reaction by contacting the 3’-end of the transcript. The structure exhibits an unusual asymmetric charge distribution which indicates the manner in which GreA interacts with the RNA polymerase elongation complex.