Levels of mRNAs encoding metabolic enzymes and their cellular activities were measured on continuous culture samples of the parasitic protists and The organisms were grown in chemostats at varying growth rates under glucose limitation or in the presence of excess glucose (EG), resulting in extensive adaptation of the cellular activities of glycolytic enzymes. rRNA and mRNA for β-tubulin were monitored as controls. In levels of all RNAs showed a biphasic dependence on growth rate (= dilution rate ), with a sharp increase at higher values. Cellular RNA levels of rate-limited by glucose decreased slightly with increasing In EG-grown cells the opposite trend was observed. Equal levels for both carbon regimes were observed at intermediate values. In both species the ratio between rRNA and mRNA encoding β-tubulin was constant, independent of the carbon regime. mRNA encoding metabolic enzymes showed varying degrees of correlation with rRNA and β-tubulin mRNA. In contrast, there was little to no correlation between mRNA levels and the activities of the enzymes they encode, even though only one of these is allosterically regulated. The data indicate that RNA levels in and are determined by growth rate and in the latter species by the availability of the carbon and energy source. Rates of synthesis of metabolic enzymes are most likely regulated at the post-transcriptional level.


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