STRUCTURE AND EXPRESSION OF SEVERAL BEAN (PHASEOLUS-VULGARIS) NUCLEARTRANSFER-RNA GENES - RELEVANCE TO THE PROCESS OF TRANSFER-RNA IMPORT INTO PLANT-MITOCHONDRIA

Bean nuclear genes for tRNA(Pro), tRNA(Thr) and tRNA(Leu) were isolate d. Expression of the tRNA(Pro) genes was demonstrated in vivo and sequ ence analysis suggested amplification of the tRNA(Pro) gene copy numbe r through duplication of a gene cluster at the same locus of the bean genome. The two tRNA(Thr) genes isolated were actively transcribed and their transcripts processed in a HeLa cell system. In vivo expression tests of these genes and aminoacylation assays of the corresponding i n vitro transcripts showed the presence of identity determinants in th e anticodon of plant tRNA(Thr). The tRNA(Leu) gene was not expressed d ue to deviation from the consensus in the internal B-box promoter. The same sequence deviation also prevented aminoacylation of the correspo nding in vitro transcript. This tRNA(Leu) however exists in plants and is synthesized from another gene with a consensus B-box promoter. Pla nt mitochondria import from the cytosol a number of nucleus-encoded tR NAs, including tRNA(Leu) and tRNA(Thr). From the available sequence da ta, we could not identify any conserved structural motif characteristi c for the nucleus-encoded tRNAs imported into plant mitochondria, eith er in the tRNAs, or in the gene flanking sequences. These results sugg est that recognition of tRNAs for import is idiosyncratic and likely t o depend on protein/RNA interactions that are specific to each tRNA or each isoacceptor group.