Unlocking Bacterial Development: A New Family of Peptides Key to Microbial Growth
- Ray Sullivan
- Jul 16
- 3 min read
Bacteria are masters of adaptation, constantly producing a diverse array of small organic molecules, natural products, to interact with their environment and mediate crucial physiological processes. Among these, surface-active peptides, or surfactants, play a role in allowing microbes to build complex aerial structures, a vital step for growth and development, especially in filamentous bacteria like Streptomyces. New research from the Seyedsayamdost Lab at Princeton unveils a unique family of these peptides, called clavusporins, shedding light on a previously underexplored aspect of microbial development.
Clavusporins are unusual and hydrophobic ribosomal peptides that stand out due to their multiple C-methylations at unactivated carbon centers. These unique modifications are crucial to their function: they drastically reduce the surface tension of water, thereby aiding in the development of Streptomyces bacteria. This function is particularly important for processes like erecting aerial hyphae and advancing sporulation and proliferation. While SapB was previously the only well-known surface-active natural product in Streptomyces development, its absence or divergence in many actinomycete genomes hinted at the existence of alternative surfactants, a gap now filled by clavusporins.
The discovery and characterization of clavusporins presented significant challenges, requiring a genetic promoter swap and codon optimization. Researchers identified an uncharacterized protein superfamily, DUF5825 (MpcC), working in conjunction with a vitamin B12-dependent radical S-adenosylmethionine (rSAM) metalloenzyme (MpcB), as the key players in synthesizing these peptides. Together, MpcB and MpcC form a novel iterative β-methyltransferase pair responsible for installing the crucial C-methylations. This methylation process is highly unusual and essential, as the unmodified core peptide (MpcA1-13, KPSVGITITVPFR) acts as a much weaker surfactant.
The impact of clavusporins on bacterial development is profound. Experiments showed that clavusporins significantly altered the shape of aqueous droplets by reducing surface tension to levels similar to saturated SDS solutions, highlighting their powerful surfactant properties.
When the genes responsible for clavusporin production (mpcB or mpcC) were inactivated in S. clavuligerus, the mutants were unable to sporulate, growing only substrate mycelia and limited aerial hyphae, a phenotype often referred to as "bald".
Conversely, supplementing these "bald" mutant strains with clavusporins restored the growth of aerial hyphae, visually and through scanning electron microscopy. This effect was highly specific; other general surfactants like surfactin or common cationic/anionic surfactants did not complement the bald phenotype and sometimes even exhibited cytotoxicity.
Interestingly, the unmodified core peptide, MpcA1-13, also had a similar effect in restoring aerial hyphae growth when externally supplemented, suggesting that while methylations boost surfactant activity and peptide stability, the core structure is functionally relevant. However, mutants lacking MpcB/MpcC are unlikely to accumulate the unmodified core peptide, explaining their developmental deficiency.
Further studies showed that clavusporins primarily function as surfactants rather than direct regulatory molecules, as their absence did not substantially alter the transcript levels of key regulatory genes involved in aerial hyphae formation or sporulation.
The significance of clavusporins extends beyond S. clavuligerus. Bioinformatic analyses revealed that the operon encoding clavusporins (termed mpc or mpg for orthologous clusters) is widespread among actinomycete bacteria. Researchers found 229 distinct mpc gene clusters, all from actinomycetes, sharing identical genomic contexts, implying a prevalent and analogous function across these diverse bacteria. This was further supported by experiments in Streptomyces ghanaensis, where inactivation of the orthologous mpgB gene also resulted in altered development and a white phenotype compared to the wild type's dark gray spores.
This discovery provides a mechanistic explanation for the long-known "bald" phenotype observed when the large plasmid in S. clavuligerus is removed, as this plasmid carries the mpc cluster. The elucidation of clavusporins' structure, biosynthesis, and function opens up rich avenues for future research into microbial development, natural product chemistry, and enzymology, suggesting that many more unknown natural products likely act as surfactants in bacterial sporulation.
Zhang C, Li Y, Overton EN, Seyedsayamdost MR. Peptide surfactants with post-translational C-methylations that promote bacterial development. Nat Chem Biol. 2025 Jul;21(7):1069-1075. doi: 10.1038/s41589-025-01882-8. Epub 2025 Apr 22. PMID: 40263466.
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