We report the identification of citrocin, a 19-amino acid-long antimicrobial lasso peptide from the bacteria Citrobacter pasteurii and Citrobacter braakii. We refactored the citrocin gene cluster and heterologously expressed it in Escherichia coli. We determined citrocin's NMR structure in water and found that is reminiscent of that of microcin J25 (MccJ25), an RNA polymerase-inhibiting lasso peptide that hijacks the TonB-dependent transporter FhuA to gain entry into cells. Citrocin has moderate antimicrobial activity against E. coli and Citrobacter strains. We then performed an in vitro RNA polymerase (RNAP) inhibition assay using citrocin and microcin J25 against E. coli RNAP. Citrocin has a higher minimal inhibition concentration than microcin J25 does against E. coli but surprisingly is ϳ100-fold more potent as an RNAP inhibitor. This suggests that citrocin uptake by E. coli is limited. We found that unlike MccJ25, citrocin's activity against E. coli relied on neither of the two proton motive force-linked systems, Ton and Tol-Pal, for transport across the outer membrane. The structure of citrocin contains a patch of positive charge consisting of Lys-5 and Arg-17. We performed mutagenesis on these residues and found that the R17Y construct was matured into a lasso peptide but no longer had activity, showing the importance of this side chain for antimicrobial activity. In summary, we heterologously expressed and structurally and biochemically characterized an antimicrobial lasso peptide, citrocin. Despite being similar to MccJ25 in sequence, citrocin has an altered activity profile and does not use the same outer-membrane transporter to enter susceptible cells.

We report the heterologous expression, structure, and antimicrobial activity of a lasso peptide, ubonodin, encoded in the genome of Burkholderia ubonensis. The topology of ubonodin is unprecedented amongst lasso peptides, with 18 of its 28 amino acids found in the mechanically bonded loop segment. Ubonodin inhibits RNA polymerase in vitro and has potent antimicrobial activity against several pathogenic members of the Burkholderia genus, most notably B. cepacia and B. multivorans, causative agents of lung infections in cystic fibrosis patients.

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Microviridins and other ω−ester linked peptides (OEPs) are characterized by sidechain-sidechain linkages installed by ATP-grasp enzymes. Here we describe the discovery of a new family of OEPs, the gene clusters of which also encode an O-methyltransferase with homology to the protein repair catalyst protein L-isoaspartyl methyltransferase (PIMT). We produced the first example of this new ribosomally synthesized and post-translationally modified peptide (RiPP), fuscimiditide, via heterologous expression. NMR analysis of fuscimiditide revealed that the peptide contains two ester crosslinks forming a stem-loop macrocycle. Furthermore, an unusually stable aspartimide moiety is found within the loop macrocycle. We have also fully reconstituted fuscimiditide biosynthesis in vitro establishing that ester formation catalyzed by the ATP-grasp enzyme is an obligate, rate-limiting first biosynthetic step. Aspartimide formation from aspartate is catalyzed by the PIMT homolog in the second step. The aspartimide moiety embedded in fuscimiditide hydrolyzes regioselectively to isoaspartate (isoAsp). Surprisingly, this isoAspcontaining protein is also a substrate for the PIMT homolog, thus driving any hydrolysis products back to the aspartimide form. Whereas aspartimide is often considered a nuisance product in protein formulations, our data here suggest that some RiPPs have aspartimide residues intentionally installed via enzymatic activity.

The antimicrobial peptide microcin J25 (MccJ25) is matured by two enzymes, McjB and McjC, from a 58 amino acid (aa) preprotein, McjA, into its final 21 aa lasso topology. Herein we have investigated the role of the leader peptide of McjA and found that only the eight C-terminal amino acids of this leader peptide are required for maturation of MccJ25. There is a high content of lysine residues in the McjA leader peptide, but herein we also demonstrate that these charged amino acids do not play a major role in the maturation of MccJ25. Alanine scanning mutagenesis studies revealed that the Thr-35 residue in the leader peptide is critical for correct processing of McjA into mature MccJ25. In the absence of detailed structural and biochemical data about McjB and McjC, these studies allow us to propose a putative role for the leader peptide as a simple motif for docking of the McjA preprotein in the maturation enzymes.