Abstract: The helper protein encoded by the phospholipase A1 accessory protein S (plaS) gene of Serratia marcescens can significantly improve the expression and activity of phospholipase A1 in Escherichia coli, but it can inhibit the growth of the host strain; however, the underlying mechanism has not been reported. The key gene regions involved in the growth inhibitory effect of PlaS were determined by bioinformatic analysis and the growth characteristics of the expression strains constructed by truncating the sequence encoding 23, 24, 25, 26 and 27 N-terminal amino acids of PlaS (dS23P28, dS24P28, dS25P28, dS26P28 and dS27P28), and the inhibitory mechanism was preliminarily investigated by scanning electron microscopy (SEM) and flow cytometry (FCM). The results indicated that dS23P28, dS24P28, dS25P28 and dS26P28 showed growth inhibition to different extents, while dS27P28 did not. Preliminary results showed that the first 27 amino acids at the PlaS N-terminal were crucial for its inhibitory effect. The results of SEM and FCM showed that the expressed PlaS caused serious damage to the cell morphology of E. coli, while this effect disappeared after truncation of 27 amino acids from the N-terminal. These results were consistent with the growth characteristics. This study provides a theoretical basis for further studies on the functions of Serratia marcescens PlaS.

Key words: Serratia marcescens; phospholipase A1 accessory protein S; Escherichia coli; growth inhibition; N-terminal truncation