Generation of Thiocillin Ring Size Variants by Prepeptide Gene Replacement and in Vivo Processing by <i>Bacillus cereus</i>

The thiocillins from <i>Bacillus cereus</i> ATCC 14579 are natural products from the broader class of thiazolyl peptides. Their biosynthesis proceeds via extensive post-translational modification of a ribosomally encoded precursor peptide. This post-translational tailoring involves a key step formal cycloaddition between two distal serine residues. In the wild-type structure, this cycloaddition forms a major macrocycle circumscribed by 26-atoms (shortest path). Results presented herein demonstrate the promiscuity of this last step by means of a set of “competition” experiments. Cyclization proceeds in many cases to provide altered ring sizes, giving access to several variant rings sizes that have not previously been observed in nature