The N-terminal domain of the vaccinia virus protein E3L (Z alpha(E3L)) is essential for full viral pathogenicity in mice. It has sequence similarity to the high-affinity human Z-DNA-binding domains Z alpha(ADAR1) and Z alpha(DLM1). Here, we report the solution structure of Z alpha(E3L) and the chemical shift map of its interaction surface with Z-DNA. The global structure and the Z-DNA interaction surface of Z alpha(E3L) are very similar to the high-affinity Z-DNA-binding domains Z alpha(ADAR1) and Z alpha(DLM1). However, the key Z-DNA contacting residue Y48 of Z alpha(E3L) adopts a different side chain conformation in unbound Z alpha(E3L), which requires rearrangement for binding to Z-DNA. This difference suggests a molecular basis for the significantly lower in vitro affinity of Z alpha(E3L) to Z-DNA compared with its homologues.