To understand the basis for tissue-specific production and accumulation of alanine tRNA in silkworms, we have examined the organization of the genes that code for silk gland-specific and constitutive alanine tRNAs. We have found that all of the silk gland-specific tRNAMa genes (-20) appear to be tightly clustered at a single locus in the Bombyx genome. These genes are arranged in tandem at intervals of --150 base pairs. In contrast to the arrangement of the silk gland-specific tRNAAll genes, most of the 20 to 30 constitutive tRNAAIa genes are dispersed in the genome. Silk gland-specific tRNAAla genes are not amplified or grossly rearranged in the silk gland. Thus it is likely that differential transcription, rather than changes in gene number or structure, accounts for the tissue-specific accumulation of tRNAAla.
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