Efrotomycin

Four new efrotomycins, A1-A4 (1-4), were isolated from the salt mine-derived Amycolatopsis cihanbeyliensis DSM 45679 and structurally determined. Efrotomycins A3 (3) and A4 (4) feature a tetrahydrofuran ring configured distinctly from known elfamycins. Heterologous expression of the efrotomycin gene cluster (efr BGC) in Streptomyces lividans SBT18 led to efrotomycin B1 (5), the yield of which was improved several fold upon introduction of the transporter gene efrT, a putative self-resistance determinant outside of the efr BGC.

Nocardia lactamdurans has been shown to catabolyse uracil via the reductive pathway. The end product of this pathway, beta-alanine, is incorporated into the pyridone ring of efrotomycin. Support for this proposal includes: (1) reversal of thymine inhibition of efrotomycin biosynthesis by dihydrouracil and N-carbamoyl-beta-aline, two intermediates of the catabolic pathway; (2) incorporation of [5,6-3H]-uracil into efrotomycin with a relative molar specific activity of approximately 0.5, close to the theoretical maximum; and (3) 13C coupling at C4 and C5 of efrotomycin after feeding resting cells with [4,5-13C]-uracil. Our results do not rule out the possibility of an alternative source of beta-alanine or the coexistence of uracil catabolism via oxidative reactions.

MDL 62,879 (formerly GE 2270 A) is a novel antibiotic active against Gram-positive bacteria by inhibiting protein synthesis. MDL 62,879 is not active against Gram-negative bacteria, but inhibits cell-free protein synthesis in extracts from Escherichia coli, and shows a high binding affinity for its elongation factor Tu (EF-Tu). We prepared ribosomes and protein-synthesis elongation factors from three sources: E. coli, Bacillus subtilis, and a strain of B. subtilis selected for resistance to MDL 62,879 (strain G1674). Homologous and heterologous reconstituted systems were used to compare the effects of MDL 62,879 and of efrotomycin, an EF-Tu inhibitor of the kirromycin class, which is inactive against both B. subtilis and E. coli. We showed that in cell-free protein synthesis: (a) E. coli was sensitive to both MDL 62,879 and efrotomycin; (b) B. subtilis was sensitive to MDL 62,879, but not to efrotomycin; (c) B. subtilis G1674 was resistant to both antibiotics. In the E. coli system and in the system from wild-type B. subtilis, inhibition by MDL 62,879 was reversed upon addition of purified EF-Tu from B. subtilis G1674. This demonstrates that the antibiotic acts by inhibition of EF-Tu. In contrast, extracts from B. subtilis failed to restore activity in an efrotomycin-inhibited E. coli system. Dominance or resistance to MDL 62,879 and of sensitivity to efrotomycin in heterologous cell-free protein synthesis confirms that inhibition of EF-Tu by the two antibiotics is mediated by different mechanisms of action.