Toyocamycin (TM) is a pyrrolopyrimidine antibiotic, which also serves as the precursor for chemical conversion to the antitumor agents sangivamycin and tricyclicnucleoside phosphate (TNP) (Fig. 1). These types of agents originally discovered as having antiviral activity were evaluated in phase II and phase III clinical trials against a variety of human cancers by the National Cancer Institute (NCI)(86).
Guanosine triphosphate (GTP) has been indicated as a possible precursor of Toyocamycin (TM) by other groups. Evaluation of salvage TM synthesis from exogenous adenine and guanine by TM strain FCRF 341 in a 120 dm3 batch fermentation (Fig. 4) indicated that de novo synthesis may be the major source of GTP for TM production and may be limiting. To alter the intracellular GTP pool levels, feedback regulation mutants of the enzymes IMP dehydrogenase and XMP aminase (Fig. 5) were sought among 8-azaguanine and decoyinine resistant populations of FCRF 341. Eight-azaguanine did not inhibit the growth of FCRF 341 but docoyinine did and a resistant strain DC-1 was isolated; however, this strain of Streptomyces crestomyceticus did not demonstrate enhanced TM or purine nucleotide production. Cloning of the parent FCRF 431 followed by UV mutagenesis, produced several survivors demonstrating improved TM production. Among these strains, a VU mutant U190 demonstrated reproducible TM antibiotic concentrations in ug/cm3 (titer) of over twice the level of the parent. Carbon catabolite repression was evaluated in both the parent and strain U190 with 2-deoxyglucose and found not to affect TM production (86, 276).
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