Pyrrolobenzodiazepines, a class of natural products produced by actinomycetes, are sequence selective DNA alkylating compounds with significant antitumor properties. Among the pyrrolo[1,4]benzodiazepines (PBDs) sibiromycin, one of two identified glycosylated PBDs, displays the highest affinity for DNA and the most potent antitumor properties. Despite the promising antitumor properties clinical trials of sibiromycin were precluded by the cardiotoxicity effect in animals attributed to the presence of the C-9 hydroxyl group. As a first step toward the development of sibiromycin analogs, we have cloned and localized the sibiromycin gene cluster to a 32.7-kb contiguous DNA region. Cluster boundaries tentatively assigned by comparative genomics were verified by gene replacement experiments. The sibiromycin gene cluster consisting of 26 open reading frames reveals a "modular" strategy in which the synthesis of the anthranilic and dihydropyrrole moieties is completed before assembly by the nonribosomal peptide synthetase enzymes. In addition, the gene cluster identified includes open reading frames encoding enzymes involved in sibirosamine biosynthesis, as well as regulatory and resistance proteins. Gene replacement and chemical complementation studies are reported to support the proposed biosynthetic pathway.

Tomaymycin produced by Streptomyces achromogenes is a naturally produced pyrrolobenzodiazepine (PBD). The biosynthetic gene cluster for tomaymycin was identified and sequenced. The gene cluster analysis reveals a novel biosynthetic pathway for the anthranilate moiety of PBDs. Gene replacement and chemical complementation studies were used to confirm the proposed biosynthetic pathway.

Pursuit of the actinomycete pyrrolobenzodiazepine natural product sibiromycin as a chemotherapeutic agent has been limited by its cardiotoxicity. Among pyrrolobenzodiazepines, cardiotoxicity is associated with hydroxylation at position 9. Deletion of the methyltransferase gene sibL abolishes the production of sibiromycin. Supplementation of growth media with 4-methylanthranilic acid can substitute for its native 3-hydroxy congener that is natively produced. Cultures grown in this fashion yielded 9-deoxysibiromycin. In this study, we characterize the structure and biological activity of sibiromycin and 9-deoxysibiromycin methyl carbinolamines. Preliminary in vitro evidence suggests that 9-deoxysibiromycin exhibits reduced cardiotoxicity while gaining antitumor activity. These results strongly support further exploration of the production and evaluation of monomeric and dimeric glycosylated pyrrolobenzodiazepine analogs of sibiromycin.

The correct representation is likely "DNA:PBD = 20:1", but requires confirmation with further experimentation. Consequently, we withdraw the Tm measurements described in the second paragraph of page 974 and the conclusion that deoxysibiromycin binds more tightly to DNA than sibiromycin. Future experiments repeating the melting point assay will be admitted into the scientific record. This correction does not change any other aspects in the interpretation of data, analysis, or the article's conclusions. The authors apologize for this unintended error.