We report the expression and characterization of a truncated form of MycA from the Mycosubtilin gene cluster from Bacillus subtilis. The MycA fragment contains a new amino transferase (AMT) tailoring domain, allowing the first detailed study of a PLP-dependent enzyme operating in cis within the PKS and NRPS biosynthetic paradigm. As the AMT domain acts on covalently bound beta-ketothioesters, and is therefore a single-turnover system, electrospray ionization-Fourier transform mass spectrometry (ESI-FTMS) was used to observe the amine-transfer reaction both for amine donor substrate specificity and to regiospecifically determine enzyme-bound intermediates. We confirm the function of the AMT domain, dissect the mechanistic steps of amine transfer, identify the preferred amine source, and localize the beta-ketothioester substrate during amine transfer.

For screening a pool of potential substrates that load carrier domains found in non-ribosomal peptide synthetases, large molecule mass spectrometry is shown to be an ideal, unbiased assay. Combining the high resolving power of Fourier-Transform Mass Spectrometry with the ability of adenylation domains to select their own substrates, the mass change that takes place upon formation of a covalent intermediate thus identifies the substrate. This assay has an advantage over traditional radiochemical assays in that many substrates, the substrate pool, can be screened simultaneously. Using proteins on the nikkomycin, clorobiocin, coumermycin A 1 , yersiniabactin, pyochelin and enterobactin biosynthetic pathways as proof-of-principle, preferred substrates are readily identified from substrate pools. Furthermore this assay can be used to provide insight into the timing of tailoring events of biosynthetic pathways as demonstrated using the bromination reaction found on the jamaicamide biosynthetic pathway. Finally, this assay can provide insight into the role and function of orphan gene clusters for which the encoded natural product is unknown. This is demonstrated by identifying the substrates for two NRPS modules from the genes pksN and pksJ, that are found on an orphan NRPS/PKS hybrid cluster from Bacillus subtilis. This new assay format is especially timely for activity screening in an era when new types of thiotemplate assembly lines that defy classification are being discovered at an accelerating rate. KeywordsElectrospray Fourier transform mass spectrometry; Non-ribosomal peptide synthetase; Polyketide synthase; Carrier domains; Post-translational modification Almost weekly, new non-ribosomal peptide synthetase (NRPS) gene clusters encoding for proteins that biosynthesize bioactive compounds are discovered (1-4). Since many of the compounds produced by non-ribosomal peptide synthetase (NRPS) as well as polyketide * To whom correspondence should be addressed, kelleher@scs.uiuc.edu, fax number 217 244-8068 and phone number 217 NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2008 October 9. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript synthase (PKS) paradigm have potent medicinal utility or are involved in virulence, and display unusual chemistry, they are of great academic and industrial interest. Therefore we wanted to develop an alternative method for substrate screening to complement the more traditional radioactive assays (5,6). In NRPS and PKS systems, the substrates and intermediates are loaded onto and processed while attached to the pantetheinyl functionality on a carrier domain (7,8).Examples of NRPS or hybrid NRPS/PKS natural products for which substrates load onto carrier domains are: the siderophore pyoverdine (9), a virulence factor excreted by pseudomonads, the antimicrobial agents penicillin (10), vancomycin (11) gramicidin (12) and the antitumor agent calicheamycin (13). With ∼300 genomes sequenced and available in the public ...