No abstract

The rifamycins specifically inhibit DNA-dependent RNA polymerase (nucleosidetriphosphate: RNA nucleotidyl transferase, DNA-dependent. E.C.2.7.7.6) of bacterial but not of mammalian origin.1' 2 Mizuno et al.3' 4 have shown that streptovaricin, a substance chemically related to the rifamycins, also specifically inhibits DNA-dependent RNA polymerase from E. coli but not from Ehrlich ascites cells. These authors demonstrated that DNA, CTP, Mg++, or Mn++, do not interact with the antibiotic. We have previously reported (a) that inhibition by the rifamycins is dependent on the amount of enzyme present,2 (b) that the sensitivity of bacterial and mammalian cells is strikingly different,2 and (c) that RNA polymerase isolated from bacteria resistant to rifamycin is not affected by the antibiotic.' All these findings suggest the occurrence of a direct interaction between enzyme and antibiotic.Using a 14C-labeled rifamycin derivative, we have now obtained evidence that RNA polymerase forms a very stable complex with the antibiotic with simultaneous loss of activity. Moreover, RNA polymerase from rifamycin-resistant mutants of E. coli does not form a complex with the antibiotic and is not inhibited.Materials and Methods.-CTP-'H was purchased from Schwarz BioResearch (Orange-

No abstract

In order to find a 3,4-dihydro-2H-naphtho[1,2-b]pyran-5,6-dione more potent than the naturally occurring 2,2-dimethyl derivative [beta-lapachone (10a)], we synthesized a series of analogous compounds with modifications at position 2 of the pyran ring or at positions 8 and 9 of the benzene ring. Of the compounds tested in vitro for inhibition of RNA-dependent DNA polymerase and in mice infected with Rauscher leukemia, all retained good enzyme activity. Inhibition of the reverse transcriptase activity of the 2,2-substituted derivatives 10b-e was as strong as 10a. However, only the 2-methyl-2-phenyl derivative 10e proved to be about as potent as the 2,2-dimethyl reference compound 10a in prolonging the mean survival time of mice with Rauscher leukemia virus induced leukemia.

Papulacandin B, the main component of a series of antibiotics produced by the deuteromycete Papularia sphaerosperma, inhibits the growth of yeasts. It is a highly amphophilic substance containing residues of glucose, galactose and two long-chain unsaturated fatty acids. It does not cause the release of potassium ions from yeast cells and therefore differs in its mode of action from the polyene antibiotics. Papulacandin B, at concentrations slightly below the minimal inhibitory concentration, partially but selectively inhibited the incorporation of glucose into cells of Saccharomyces cerevisiae and Candida albicans. A much clearer effect was observed on spheroplasts, obtained by digestion of the yeast cell wall with snail digestive enzyme. Incubation of spheroplasts in a minimal medium stabilised by sorbitol led to the incorporation of glucose into alkali-insoluble glucan, mannan and a fraction containing mainly alkali-soluble 1,3-glucan together with some glycogen. Papulacandin B inhibited the synthesis of the alkali-insoluble fraction, while causing a slight stimulation of glucose incorporation into the other two polysaccharide fractions. The 50 % inhibitory concentrations of papulacandin B for glucan synthesis in S. cerevisiae spheroplasts and C. albicans spheroplasts were respectively 0.16 pg/ml and 0.03 pg/ml. C. albicans cells were irradiated with ultraviolet light and selected for maximum resistance to papulacandin B. The 50 % inhibitory concentration for glucan synthesis in spheroplasts prepared from this mutant was 2.5 pg/ml. Echinocandin B, a polypeptide antibiotic containing a long-chain fatty acid, also inhibited the synthesis of glucan in spheroplasts. It is concluded that papulacandin B and probably also echinocandin B inhibit glucan synthesis during cell-wall synthesis, and thus cause lysis of cells by osmotic rupture.