In order to study the signal molecule synthase gene expI of Dickeya dadantii, the gene expI was amplified from the genome of the type strain DSM–18020. Bioinformatics analysis showed that there was a complete open reading frame (ORF) in DSM–18020 expI, which was 639 bp in length and had 99% homology to N–acyl–homoserine lactone (AHLs) synthetase of D. dadantii 3937. The molecular weight of the protein encoded by DSM–18020 expI is 24,704, and the theoretical isoelectric point is 5.85. The hydrophobic amino acids are more abundant than hydrophilic amino acids and distributed evenly throughout the peptide chain, with no signal peptide. expI gene was cloned into the expression vector pET28α and transformed into Escherichia coli BL21 (DE3). The expression product was basically identical with the expectation.