Glycopeptides

Glycopeptide antibiotic resistance genes in glycopeptide-producing organisms

The mechanism of high-level resistance to vancomycin in enterococci consists of the synthesis of peptidoglycan terminating in D-alanyl-D-lactate instead of the usual D-alanyl-D-alanine. This alternate cell wall biosynthesis pathway is ensured by the collective actions of three enzymes: VanH, VanA, and VanX. The origin of this resistance mechanism is unknown. We have cloned three genes encoding homologs of VanH, VanA, and VanX from two organisms which produce glycopeptide antibiotics: the A47934 producer Streptomyces toyocaensis NRRL 15009 and the vancomycin producer Amycolatopsis orientalis C329.2. The predicted amino acid sequences are highly similar to those found in VRE: 54 to 61% identity for VanH, 59 to 63% identity for VanA, and 61 to 64% identity for VanX. Furthermore, the orientations of the genes, vanH, vanA, and vanX, are identical to the orientations found in vancomycin-resistant enterococci. Southern analysis of total DNA from other glycopeptide-producing organisms, A. orientalis 18098 (chloro-eremomycin producer), A. orientalis subsp. lurida (ristocetin producer), and Amycolatopsis coloradensis subsp. labeda (teicoplanin and avoparcin producer), with a probe derived from the vanH, vanA, and vanX cluster from A. orientalis C329.2 revealed cross-hybridizing DNA in all strains. In addition, the vanH, vanA, vanX cluster was amplified from all glycopeptide-producing organisms by PCR with degenerate primers complementary to conserved regions in VanH and VanX. Thus, this gene sequence is common to all glycopeptide producers tested. These results suggest that glycopeptide-producing organisms may have been the source of resistance genes in vancomycin-resistant enterococci.

Biosynthesis of acylpeptidolactones of the daptomycin type. A comparative analysis of peptide synthetases forming A21978C and A54145

A21978C and A54145 are antibacterial 13-residue peptides with a medium-chain-acylated amino terminus and a 10-residue lactone ring; they are produced by strains of Streptomyces roseosporus and Streptomyces fradiae, respectively. The structural differences in their peptide chains, which include amino acid replacements and modifications (L-Glu2-->L-Asn, L-Asn(OH)3-->L-Asp, Sar5-->Gly, L-Ala6-->L-Orn, L-Lys8-->D-Ala, L-Asp(OMe)9-->L-Asp, L-Asn11-->D-Ser, and L-lle13-->L-Kyn; Sar = sarcosine; L-Orn = L-ornithine, L-Kyn = L-kynurenine), reside in the multienzymatic templates directing their biosynthesis. We have examined the peptide synthetases employing immunodetection and substrate activation detected by the amino-acid-dependent ATP-PP1-exchange reaction. Two different antibodies specific for actinomycin synthetase 2 and a peptide sequence characteristic of acyl-CoA-synthetases/peptide synthetases were applied. For the A21978 system two peptide synthetases of 670 and 240 kDa were detected, together with two similar proteins of 630 and 440 kDa occurring in varying amounts. The latter are suggested to be degradation products of an unstable multienzyme. Activation of L-Asp, L-Thr, Gly, L-Orn, L-Ala and L-Ser were assigned to the high-molecular-mass components of 670, 630 and 440 kDa. The 240-kDa protein was purified to homogeneity and shown to catalyse activation of L-kynurenine. The A54145 system consisted of three peptide synthetases of 690, 590 and 250 kDa. Activations of L-Asn. L-Thr and Gly were found. The 250-kDa synthetase was capable of activating isoleucine and valine. Both systems thus show a comparable organisation; implications for the modular construction of their peptide synthetases are discussed.