Molecular cloning and expression of Salmonella paratyphi A 52 kDa specific protein gene

Monoclonal antibodies (MAbs) specific to Salmonella paratyphi A have been established by our group in 1989. These MAbs were proven to be species-specific for 52 kDa protein of S. paratyphi A but the nature of this protein is unknown. However, our group have proved that the 52 kDa protein which is specific to S. typhi was flagellin. This present study has characterized the 52 kDa protein of S. paratyphi A and identified its encoded gene. The plasmid containing the specific 52 kDa antigen gene was cloned from the S. paratyphi A genome, herein designated pSKA-4. Partial nucleotide sequences from this clone was analysed by computer program and found to be phase 1-a flagellin gene of S. paratyphi A. In addition, the nucleotide sequence analysis from such clone also showed that the structural gene for phase 1 flagellin has amino acid sequences conserved at the terminal whereas the central region is variable among Salmonella spp. Therefore, the central portion of flagellin which highly polymorphic in amino acid sequences would be the most specific to S. paratyphi A, thus, should be used as specific antigen for developing specific diagnosis of S. paratyphi A infection. Using the PCR technique, an expression plasmid containing the antigen gene producing only the variable region in the central portion of flagellin from S. paratyphi A, namely pSKA-7, has been established. The recombinant protein produced by the established plasmid has a MW 33.5 kDa as detected by immunoblotting using specific MAbs. Further study by using this specific flagellin protein for immunodiagnosis of S. paratyphi A infection is being carried out in our laboratory.