We describe the selective irreversible inhibition of mengovirus growth in cultured cells by a combination of two
pyrrolopyrimidine nucleoside analogues, 5-bromotubercidin (BrTu) and
tubercidin (Tu). At a concentration of 5 microgram/ml, BrTu reversibly blocked the synthesis of cellular
mRNA and rRNA but did not inhibit either mengovirus
RNA synthesis or multiplication. BrTu is a potent inhibitor of
adenosine kinase, and low concentrations of BrTu (e.g., 0.5 microgram/ml), which did not by themselves inhibit cell growth, blocked phosphorylation of Tu and thus protected uninfected cells against irreversible cytotoxicity resulting from Tu incorporation into
nucleic acids. In contrast, in mengovirus-infected cells, BrTu did not completely inhibit Tu incorporation into mengovirus
RNA, allowing the formation of Tu-containing functionally defective
polynucleotides that aborted the virus development cycle. This increased incorporation of Tu coupled to mengovirus
infection could be attributed either to a reduction in the inhibitory action of BrTu and/or its
nucleotide derivatives at the level of
nucleoside and
nucleotide kinases and/or, perhaps, to an effect upon the
nucleoside transport system. The virus life cycle in
nucleoside-treated cells progressed to the point of synthesis of negative strands and probably to the production of a few defective new positive strands. Irreversible virus growth arrest was achieved if the
nucleoside mixture of BrTu (0.5 to 10 microgram/ml) and Tu (1 to 20 microgram/ml) was added no later than 30 min after
virus infection and maintained for periods of 2 to 8 h. The cultures thus "cured" of mengovirus
infection could be maintained and transferred for several weeks, during which they neither produced detectable virus nor showed a visible cytopathic effect; however, the infected and cured cells themselves, while metabolically viable, were permanently impaired in
RNA synthesis and unable to divide. Although completely resistant to superinfecting picornaviruses, they retained the ability to support the growth of several other viruses (vaccinia virus, reovirus, and
vesicular stomatitis virus), showing that cured cells had, in general, retained the metabolic and structural machinery needed for virus production. The resistance of cured cells to
superinfection with picornaviruses seemed attributable neither to
interferon action nor to destruction or blockade of
virus receptors but more likely to the consumption of some host factor(s) involved in the expression of early viral functions during the original
infection.