Characterization of the syringomycin synthetase gene cluster - A link between prokaryotic and eukaryotic peptide synthetases

With this work we have completed the characterization of the syringomycin s ynthetase gene cluster. In particular, by sequencing additional 28.5 kiloba se pairs we show that the nine modules involved in the binding of the nine amino acids of syringomycin are localized on SyrB and SyrE, with SyrE carry ing eight modules. The recombinant SyrE and the first and second modules of SyrE (SyrE1 and SyrE2) have been expressed in Escherichia coli and purifie d. The biochemical data indicate that SyrB binds threonine, the putative pr ecursor of the last amino acid of syringomycin, whereas SyrE1 and SyrE2 bin d serine, the first and the second amino acids of syringomycin, respectivel y. On the basis of the sequence analysis and the biochemical data presented here, it appears that syringomycin synthetase is unique among peptide synt hetases in that its genetic organization does not respect the "colinearity rule" according to which the order of the amino acid binding modules along the chromosome parallels the order of the amino acids on the peptide. This feature, together with the absence of a single transcription unit and the a bsence of epimerase-like domains make syringomycin synthetase more related to the eukaryotic peptide synthetases than to the bacterial counterparts.