A family of non-allelic tRNA(ValGUU) genes from the cellular slime mold Dictyostelium discoideum

Abstract

A haploid genome of the cellular slime mold Dictyostelium discoideum contains at least 14 non-allelic gene copies coding for a tRNA(ValGUU). The structure, genomic organization, and expression of these genes have been analyzed in relation to stages of the developmental cycle. So far, 13 tRNA(ValGUU) genes have been isolated and characterized. All genes contain identical mature tRNA-coding regions, and consequently identical gene internal promoter elements. However, different genes differ with respect to their 5'- and 3'-flanking regions, although a certain degree of sequence conservation seems apparent. Different members of this tRNA gene family appear to be randomly dispersed along the seven D. discoideum chromosomes, and not clustered at any one genomic location. In vivo expression of individual genes was studied in yeast. All but one tRNA(ValGUU) gene are actively transcribed, though with different efficiencies. There is also evidence that not all of these tRNA genes are constitutively transcribed in Dictyostelium throughout the developmental cycle. One characteristic primary transcript can only be detected in cells of the late preaggregation phase, whereas growing cells, cells in the stationary phase or cells harvested 4 h after the onset of development do not seem to carry this transcript. This product seems to be transcribed from a gene of an unusual structure. Although this particular gene has not yet been isolated, it can be predicted from the sequence of the cDNA synthesized from primary transcription products of this putative gene, that it is composed of nt 1-54 of a 3'-truncated tRNA(ValGUU) gene linked to a bona fide tRNA(ValGUU) gene.