Reduced Aminoacylation of Asparagine-Transfer RNA Early in the Developmental Cycle ofDictyostelium discoideum:Modification Pattern and Possible Significance of the Uncharged Isoacceptor tRNA3Asn

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Abstract
D. discoideum was used as a model system to elucidate the possible involvement of tRNAAsn in developmental processes. Vegetative amoebae were induced to undergo developmental transition by nutrient starvation. Since amino acid starvation alone is a specific stimulus initiating development and unacylated tRNA might be involved in control mechanisms of protein synthesis, the level of aminoacylation of tRNAAsn isoacceptors was investigated. As early as 2 min after the onset of development, the aminoacylation of tRNAAsn specifically was reduced to .apprx. 30%; at the same time 10 other tRNA species were charged normally, i.e., to 70-100%. One of the 2 major isoacceptors, tRNA3Asn, was completely deacylated; tRNA2Asn, accounted for the residual aminoacylation. Analyses of the modified nucleosides of highly purified tRNA2Asn and tRNA3Asn respectively, show that both isoacceptors are identical in their modification patterns except for the modification at the 1st position of the anticodon: tRNA2Asn comprises queuine (Q), 7-[(4,5-cis-dihydroxy-2-cyclopenten-1-ylamino)methyl]-7-deazaguanine; tRNA3Asn contains guanine.