Abstract: Phospholipase A1 (plaA) is an enzyme that hydrolyzes phospholipids. We used bioinformatics methods to analyze plaA and its accessory protein gene (plaS), which will provide a basis for further study of phospholipase. We cloned and sequenced the plaA and plaS of Serratia marcescens, and the characteristics of nucleotide and amino acid sequence were analysed. Furthermore, we also predicted the secondary structure and tertiary structure of PlaA and PlaS protein. Phylogenetic trees were reconstructed using maximum parsimony (MP), maximum likelihood (ML) and bayesian inference (BI) methods. The results showed that the dN / dS of plaS was higher than that of plaA, suggesting that the selection pressure is stronger and the evolution rate is slower. The secondary structure of PlaA and PlaS protein were mainly composed of α-helix and random coil. Bootstrap values of 16S tree were lower than that of trees referred from two genes (plaA + plaS ). Based on the phylogenetic tree which constructed by plaA，plaS and 16S, both Serratia nematodiphila and S. ureilytica were clustered together with S. marcescens. The two strains of Serratia may be more related to S. marcescens.

Key words: phospholipase A1, accessory protein gene plaS, Serratia, phylogeny