Dictyostelium discoideum: a useful model system to evaluate the function of queuine and of the Q-family of tRNAs

Abstract

The nucleoside queuosine (Q) is found in the first position of the anticodon of tRNATy r, tRNA His, tRNAAsp and tRNA Asn of eubacteria and eukaryotes except yeast [1]. In eukaryotes queuine is inserted into the tRNAs in exchange for guanine by the enzyme queuine: guanine tRNAtransglycosylase [2,3]. Experiments with germ-free mice, fed on queuine-free diet, showed that mice do not synthesize queuine de novo [4,5]. The modification queuine in the Q-family of tRNA was found to be incomplete in tumours from various sources and to change in cells and tissues undergoing differentiation or ageing [6-10]. The role of queuine in these processes is unknown. Here, we show that Dictyostelium discoideum can be applied as a model system to evaluate the biological significance of queuine. When myxamoebae of the axenic strain AX-2 are grown in a well-defined synthetic medium supplemented with yeast extract but without peptone their tRNAs comprise guanine in place of queuine. The addition of queuine to the medium, at 10 -7 M, causes an almost complete modification of the corresponding tRNAs. Analysis of tRNA from the wildtype strain, NC-4, that needs E. coli B/r for growth indicates that bacteria supply D. discoideum with queuine.