Bacterial membrane transport of β-exotoxin, an anti-metabolite of RNA synthesis

Abstract

β-Exotoxin is produced extracellularly by certain strains of Bacillus thuringiensis during the stationary growth phase, apparently resulting from nucleic acid metabolism¹. It is a monophosphorylated adenine nucleoside derivative^2,3 and can modify RNA polymerase activity of mammalian and bacterial origin in vitro through substrate competitive inhibition^4,5. RNA polymerase in nuclei from rat liver⁶ and insect larval fat bodies⁷ is sensitive to exotoxin, and inhibition seems to be subject to reversal by ATP. Other functional enzymes that catalyse reactions involving ribonucleoside triphosphates are similarly inhibited by exotoxin, such as adenylate cyclase⁸. Exotoxin is more toxic in mammalian and other eukaryotic systems than in cell-free prokaryotic extracts, and it can cross cell and nuclear membranes in mammalian systems^5,6,9. But I know of no report of any effect of exotoxin on intact, physiologically normal bacterial cells. Therefore I have investigated the intracellular effect of exotoxin on RNA synthesis in actively metabolising bacterial cultures. This report describes the intracellular inhibition of RNA synthesis by exotoxin in cultures of Escherichia coli and Bacillus subtilis treated to increase their permeability. Further data revealed that exotoxin is totally excluded by the bacterial membrane, even in those treated cells.