A new aminoglycoside antibiotic, substance AC4437

Pathway engineering in secondary metabolite-producing actinomycetes

Actinomycetes represent the microbial group richest in production of variable secondary metabolites. These mostly bioactive molecules are the end products of complex multistep biosynthetic pathways. Recent progress in the molecular genetics and biochemistry of the biosynthetic capacities of actinomycetes enables first attempts to redesign these pathways in a directed fashion. However, in contrast to several examples of designed biochemical improvement of primary metabolic processes in microorganisms, none of the products or strains derived from pathway engineering in actinomycetes discussed herein have reached pilot or production scale. The main reasons for this slow progress are the complicated pathways themselves, their complex regulation during the actinomycete cell cycle, and their uniqueness, as most pathways and products are specific for a strain rather than for a given species or larger taxonomic group. However, the modular use of a minimum of very similar enzymes and their conversion of similar intermediates to form the building blocks for the production of a maximum of divergent end products gives hope for the future application of these genetic models for the redesign of complex pathways for modified or new natural products. Several strategies that can be followed to reach this aim are discussed, mainly for the variable 6-deoxyhexose metabolism as an ubiquitously applicable example.

Detection of novel secondary metabolites

The study of antibiotics and other fermentation products has shown that a seemingly unlimited number of compounds with diverse structures are produced by microorganisms. The continued high rate of discovery of new chemical entities, in the light of the abundance of microbial products already described, is due to creative screening procedures that incorporate such features as the emphasis on unusual microorgnaisms, their special propagation and fermentation requirements, supersensitive and highly selective assays, genetic engineering both for the biosynthesis of new compounds and in the development of screening systems, early in vivo evaluation, improved isolation techniques, modern procedures for structure determination, computer-assisted identification, and an efficient multidisciplinary approach. This review focuses on the genesis and development of the gamut of methodologies that have led to the successful detection of the wide variety of novel secondary metabolites that include antibacterial, antigungal, antiviral and antitumour antibiotics, enzyme inhibitors, pharmacologically and immunologically active agents, products useful in agriculture and animal husbandry, microbial regulators, and other compounds for which no bioactive role has yet been found.

A second streptomycin resistance gene from Streptomyces griseus codes for streptomycin-3"-phosphotransferase. Relationships between antibiotic and protein kinases

Two genes, aphE and orf, coding for putative Mr 29,000 and Mr 31,000, proteins respectively, were identified in the nucleotide sequence of a 2.8 kbp DNA segment cloned from Streptomyces griseus N2-3-11. The aphE gene expressed streptomycin (SM) resistance and a SM phosphorylating enzyme in S. lividans strains. The two genes were found to be in opposite direction and seemed to share a common region of transcription termination. The aphE gene shows significant homology to the aph gene, encoding aminoglycoside 3'-phosphotransferase, APH(3'), from the neomycin-producing S. fradiae. The enzymatic specificity of the aphE gene product was identified to be SM 3"-phosphotransferase, APH(3"). The primary structure of the APH(3") protein is closely related to the members of the APH(3') family of enzymes. However, the APH(3") enzyme did not detectably phosphorylate neomycin or kanamycin. There is only low similarity of the protein to the APH(6) group of SM phosphotransferases. An evolutionary relationship between antibiotic and protein kinases is proposed.