Lantibiotic Pep5

Pep5 is a tricyclic peptide antibiotic which contains the unusual amino acids dehydrobutyrine, lanthionine and 3-methyllanthionine. It is matured from a 60-amino-acid precursor peptide (pre-Pep5) deduced from the sequence of the structural gene pepA. To study the biosynthesis of Pep5 we tried to isolate the primary translation product. We identified a peptide in crude extracts of the Pep5-producing Staphylococcus epidermidis strain using antibodies raised against a synthetic 26-residue peptide representing the leader peptide region of pre-Pep5. The putative precursor was purified by reversed-phase HPLC. The isolated peptide did not react with antibodies directed against a C-terminal fragment of mature Pep5 containing two sulfide bridges. Neither lanthionine nor 3-methyllanthionine was detected in amino acid analysis of the isolated precursor. Its amino acid sequence was identical with the sequence predicted from pepA, but Edman degradation stopped at the first threonine residue of the prolantibiotic region indicating a posttranslational modification at this position. The molecular mass of the isolated peptide was 6575.4 +/- 1.7 Da, determined by ion-spray mass spectrometry. This is in agreement with a molecule being dehydrated at the four threonine and the two serine residues in the propeptide region; such a peptide has a calculated molecular mass of 6576.7 Da. The results strongly suggest that maturation of the lantibiotic Pep5 is initiated by selective dehydration of hydroxyamino acids in the propeptide region of the primary translation product and that thioether ring formation is not closely linked to dehydration.

A wobbled 14-mer oligonucleotide was derived from the amino acid sequence of the 34-residue propeptide of the lantibiotic Pep5 (Kellner et al. 1989). Using this hybridization probe, the structural gene of Pep5, pepA, was located on the 18.6 kbp plasmid pED503. The nucleotide sequence of pepA codes for a prepeptide with 60 residues and proves that Pep5 is ribosomally synthesized. The N-terminus of the prepeptide has a high alpha-helix probability and a characteristic proteolytic cleavage site precedes the C-terminal 34-residue propeptide. Our present theory is that maturation of Pep5 involves (a) enzymic conversion of Thr, Ser and Cys into dehydrated amino acids and sulfide bridges, (b) membrane translocation and cleavage of the modified prepeptide.

The lantibiotic Pep5 is produced by Staphylococcus epidermidis 5. Pep5 production and producer immunity are associated with the 20-kb plasmid pED503. A 1.3-kb KpnI fragment of pED503, containing the Pep5 structural gene pepA, was subcloned into the Escherichia coli-Staphylococcus shuttle vector pCU1, and the recombinant plasmid pMR2 was transferred to the Pep5- and immunity-negative mutant S. epidermidis 5 Pep5- (devoid of pED503). This clone did not produce active Pep5 but showed the same degree of insensitivity towards Pep5 as did the wild-type strain. Sequencing of the 1.3-kb KpnI-fragment and analysis of mutants demonstrated the involvement of two genes in Pep5 immunity, the structural gene pepA itself and pepI, a short open reading frame upstream of pepA. To identify the 69-amino-acid pepI gene product, we constructed an E. coli maltose-binding protein-PepI fusion clone. The immunity peptide PepI was detected in the soluble and membrane fractions of the wild-type strain and the immune mutants (harboring the plasmids pMR2 and pMR11) by immunoblotting with anti-maltose-binding protein-PepI antiserum. Strains harboring either pepI without pepA or pepI with incomplete pepA were not immune and did not produce PepI. Washing the membrane with salts and EDTA reduced the amount of PepI in this fraction, and treatment with Triton X-100 almost completely removed the peptide. Furthermore, PepI was hydrolyzed by proteases added to osmotically stabilized protoplasts. This suggests that PepI is loosely attached to the outside of the cytoplasmic membrane. Proline uptake and efflux experiments with immune and nonimmune strains also indicated that PepI may act at the membrane site.