Streptolysin S (SLS) is a post-translationally
modified peptide cytolysin that is produced by the human
pathogen Streptococcus pyogenes. SLS belongs to a large family
of azole-containing natural products that are biosynthesized via
an evolutionarily conserved pathway. SLS is an important
virulence factor during S. pyogenes infections, but despite an
extensive history of study, further investigations are needed to
clarify several steps of its biosynthesis. To this end, chemical
inhibitors of SLS biosynthesis would be valuable tools to
interrogate the various maturation steps of both SLS and
biosynthetically related natural products. Such chemical
inhibitors could also potentially serve as antivirulence
therapeutics, which in theory may alleviate the spread of
antibiotic resistance. In this work, we demonstrate that FDAapproved
HIV protease inhibitors, especially nelfinavir, block a key proteolytic processing step during SLS production. This
inhibition was demonstrated in live S. pyogenes cells and through in vitro protease inhibition assays. A panel of 57 nelfinavir
analogs was synthesized, leading to a series of compounds with improved anti-SLS activity while illuminating structure−activity
relationships. Nelfinavir was also found to inhibit the maturation of other azole-containing natural products, namely those
involved in listeriolysin S, clostridiolysin S, and plantazolicin production. The use of nelfinavir analogs as inhibitors of SLS
production has allowed us to begin examining the proteolysis event in SLS maturation and will aid in further investigations of the
biosynthesis of SLS and related natural products.