Biosynthesis of Oxytetracycline by Streptomyces rimosus: Past, Present and Future Directions in the Development of Tetracycline Antibiotics

Abstract

Natural tetracycline (TC) antibiotics were the first major class of therapeutics to earn the distinction of 'broad-spectrum antibiotics' and they have been used since the 1940s against a wide range of both Gram-positive and Gram-negative pathogens, mycoplasmas, intracellular chlamydiae, rickettsiae and protozoan parasites. The second generation of semisynthetic tetracyclines, such as minocycline and doxycycline, with improved antimicrobial potency, were introduced during the 1960s. Despite emerging resistance to TCs erupting during the 1980s, it was not until 2006, more than four decades later, that a third--generation TC, named tigecycline, was launched. In addition, two TC analogues, omadacycline and eravacycline, developed via (semi)synthetic and fully synthetic routes, respectively, are at present under clinical evaluation. Interestingly, despite very productive early work on the isolation of a Streptomyces aureofaciens mutant strain that produced 6-demethyl-7-chlortetracycline, the key intermediate in the production of second- and third-generation TCs, biosynthetic approaches in TC development have not been productive for more than 50 years. Relatively slow and tedious molecular biology approaches for the genetic manipulation of TC-producing actinobacteria, as well as an insufficient understanding of the enzymatic mechanisms involved in TC biosynthesis have significantly contributed to the low success of such biosynthetic engineering efforts. However, new opportunities in TC drug development have arisen thanks to a significant progress in the development of affordable and versatile biosynthetic engineering and synthetic biology approaches, and, importantly, to a much deeper understanding of TC biosynthesis, mostly gained over the last two decades.

Keywords: Streptomyces; Streptomyces aureofaciens; Streptomyces rimosus; antibiotics; biosynthesis; chlortetracycline; oxytetracycline; polyketide synthase; polyketides; tetracyclines.

Fig. 1

Natural tetracyclines produced by different Streptomyces species. The upper and the lower peripheral regions of the tetracycline backbone are shadowed (2)

Fig. 2

Second and third generations of tetracyclines. Omadacycline and eravacycline are currently undergoing clinical evaluation

Fig. 3

Selected aromatic polyketide metabolites displaying diverse biological activities. Their corresponding gene clusters present rich source of biosynthetic enzymes, which can be readily used in biosynthetic engineering approaches for the generation of new derivatives

Fig. 4

Schematic presentation of: a) the gene cluster encoding for OTC biosynthesis from S. rimosus. The enzymatic activity of each putative gene is marked with different symbols and corresponds to the legend. The function of each gene product in the OTC gene cluster (in a) corresponds to the proposed OTC biosynthetic pathway (in b); and b) the biosynthetic pathway of oxytetracycline (OTC) and chlortetracycline (CTC), based on the biosynthetic gene clusters of OTC and CTC from S. rimosus and S. aureofaciens, respectively. The position on the tetracycline backbone of each proposed tetracycline precursor modified by the corresponding enzyme from the OTC (CTC) enzyme complex at each proposed biosynthetic step is shadowed. For colour version see: www.ftb.com.hr