Multiple controls on the intracellular trapping of uridine

Mechanism of apparent transcription inhibition by methyl mercury in cerebellar neurons

We have investigated the mechanism of inhibition of RNA synthesis by methyl mercury (MeHg) in isolated neonatal rat cerebellar cells. Each of the three component steps involved in the incorporation of exogenous [3H]uridine into cellular RNA was examined separately in whole-cell and/or subcellular preparations. Nuclear RNA polymerase activity was measured in preparations containing both free nuclei and whole cells. Incorporation of [3H]UTP into nuclear RNA was found to be unimpaired at concentrations of MeHg that inhibited whole-cell incorporation of [3H]uridine by greater than 75%. Cellular uptake of [3H]uridine was assayed in cerebellar cells treated with KCN to deplete ATP levels and block subsequent phosphorylation reactions of transported uridine. Uptake activity under these conditions was unaffected by MeHg. Measurement of intracellular phosphorylation of [3H]uridine indicated that inhibition of this activity closely paralleled that of RNA synthesis. Quantitation of individual uridine nucleotides by polyethyleneimine-cellulose TLC revealed reduced levels of UTP and UDP whereas levels of UMP were elevated, suggesting that impairment of phosphorylation was not the result of cellular ATP depletion but, more likely, a direct effect on phosphouridine kinase enzymes. This mechanism of MeHg-induced inhibition of RNA synthesis was confirmed by assays of uridine phosphorylation using cell-free extracts in which exogenous ATP was supplied.