Differential turnover of tRNAs of the queuosine family in Dictyostelium discoideum and its possible role in regulation.

In its natural environment the protist Dictyostelium discoideum grows on bacteria and queuosine-containing tRNAs of the bacteria serve as source of the nutrient factor queuine. This deazaguanine derivative is inserted into tRNAAsp, tRNAAsn, tRNAHis and tRNATyr of the amoebae. The axenic strain AX-2 of D. discoideum grows equally well in a defined medium with or without exogenous queuine. When queuine is omitted, changes occur in lactate levels, lactate dehydrogenase patterns and cytochromes and the amoebae cannot differentiate after a metabolic stress. In this report we show that growing cells contain two-fold higher levels of tRNAAsp and tRNATyr when sufficiently supplied with queuine, than those lacking queuine. In tRNAAsp a new, as yet unidentified, derivative of queuine has been discovered. When RNA synthesis is totally inhibited by actinomycin, tRNAAsp and tRNATyr remain stable in queuine-containing, but not in queuine-lacking cells. In contrast, tRNAAsn and tRNAHis become partially degraded in both conditions. We suggest that free queuine can be obtained from endogeneous tRNA and that differential salvage of queuine by tRNAs of the Q-family plays a role in the regulation of genes encoding components of redox chains.