- Among the myriad of naturally occurring bioactive compounds are the aminocyclopentitol-containing natural products that represent a family of sugar-derived microbial secondary metabolites, such as the antibiotics pactamycin, allosamidin, and trehazolin. Pactamycin, a structurally unique aminocyclitol antibiotic isolated from Streptomyces pactum, consists of a 5-membered ring aminocyclitol (cyclopentitol) unit, two aromatic rings (6-methylsalicylic acid (6-MSA) and 1-(3-Amino-phenyl)-ethanone or 3-aminoacetophenone) and a 1,1-dimethylurea. It has pronounced antibacterial, antitumor, antiviral, and antiplasmodial activities, but its development as a clinical drug was hampered by its broad cytotoxicity. Efforts to modulate its pharmacological and toxicity properties by structural modifications using synthetic organic chemistry have been difficult due to the complexity of its chemical structure.
As part of our ongoing studies on the biosynthesis of aminocyclitol-derived bioactive natural products, we have identified the biosynthetic gene cluster of pactamycin in S. pactum ATCC 27456, which paves the way for a better understanding of pactamycin biosynthesis and generating novel pactamycin analogs through biosynthetic engineering.
Through gene inactivations, feeding experiments, and in vitro enzymatic assay, we studied the biosynthesis of pactamycin, which include the modes of formation of the unique cyclopentitol unit, the 3-aminoacetophenone and the 6-methyl salicylic acid moieties. Armed with the tools needed to genetically engineer target strains of S. pactum, we were able to produce novel analogs of this untapped-class of natural products. TM-026 was generated from a ΔptmH (a radical SAM C-methyltransferase gene) mutant, whereas TM-025 was generated from a ΔptmH/ΔptmQ (a polyketide synthase gene) double knockout mutant. Both compounds show potent antimalarial activity, but lack significant antibacterial activity, and are about 10-30 times less toxic than pactamycin toward mammalian cells. The results suggest that distinct ribosomal binding selectivity or new mechanism(s) of action may be involved in their plasmodial growth inhibition, which may lead to the discovery of new antimalarial drugs and identification of new molecular targets within malarial parasites.
TM-035 was also isolated from a ΔptmH mutant. However, we found that TM-035 showed no activity against bacteria, malarial parasites, and most tested mammalian cells, but it has potent growth inhibitory activity against two well-established human head and neck squamous cell carcinomas (SCC025 and SCC104) (IC₅₀ 725 nM) in an in vitro assay. More intriguingly, the compound is significantly less active against human primary epidermal keratinocytes (HPEK), demonstrating an interesting biological phenomenon and outstanding cell type selectivity, which may lead to the development of new anticancer chemotherapy.
The production yield of pactamycin and its congeners under laboratory conditions is relatively low. This has hampered both mechanistic and preclinical studies of these promising compounds. To deepen our understanding of pactamycin biosynthesis and engineer mutant strains with improved production yields, we investigated pathway specific regulatory genes, ptmF and ptmE. Based on gene inactivation and RT-PCR studies, we found that the PtmF-PtmE system controls the transcription of the whole biosynthetic gene cluster. The results provide important insight into regulation of pactamycin biosynthesis and will contribute to future studies that aim at engineering high producing strains of S. pactum.