Heterologous expression and characterization of the exopolysaccharide fromStreptococcus thermophilus Sfi39

The genes responsible for exopolysaccharide (EPS) synthesis in Streptococcu s thermophilus Sfi39 were identified on a 20-kb genomic fragment. The two g enes, epsE and epsG, were shown to be involved in EPS synthesis as their di sruption lead to the loss of the ropy phenotype. Several naturally selected nonropy mutants were isolated, one acquired an insertion sequence (IS)-ele ment (IS905) in the middle of the eps gene cluster. The eps gene cluster wa s cloned and transferred into a nonEPS-producing heterologous host, Lactoco ccus lactis MG1363. The EPS produced was shown by chemical analysis and NMR spectroscopy to be identical to the EPS produced by S. thermophilus Sfi39. This demonstrated first that all genes needed for EPS production and expor t were present in the S. thermophilus Sfi39 eps gene cluster, and second th at the heterologous production of an EPS was possible by transfer of the co mplete eps gene cluster alone, provided that the heterologous host possesse d all necessary genetic information for precursor synthesis.