tRNA (Transfer RNA) is a small RNA (Ribonucleic Acid) chain (74-93 nucleotides) that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has a site for amino acid attachment and ...a three-base region called the anticodon that recognizes the corresponding three-base codon region on mRNA via complementary base pairing. Each type of tRNA molecule can be attached to only one type of amino acid, but because the genetic code is degenerate - that is, it contains multiple codons that specify the same amino acid - multiple types of tRNA molecules bearing different anticodons may carry the same amino acid. tRNA has primary structure (the order of nucleotides from 5' to 3'), secondary structure (usually visualized as the cloverleaf structure), and tertiary structure (all tRNAs have a similar L-shaped 3D structure that allows them to fit into the P and A sites of the ribosome). tRNA gene promoters consist of 2 separated 10bp elements (Box A and B) located downstream of the transcription start site. TFIIIC (General Transcription Factor-IIIC) binds to Box B. After binding to TFIIIC, Box A orients TFIIIC towards the start site. TFIIIC causes correct positioning of TFIIIB (Pre-Initiation Complex), which then recruits RNA Pol-III. This DNA/TFIIIB (General Transcription Factor-IIIB) / Pol III initiation Complex is very stable and may pass through many rounds of tRNA transcription. Transcription and elongation start immediately after assembly of the Initiation Complex. RNA Pol-III moves along the DNA and matches the DNA nucleotides with a complementary RNA nucleotide to create a new RNA molecule that is patterned after the DNA. The copying of the DNA continues until the RNA Pol-III reaches a termination signal, which is a specific set of nucleotides that mark the end of the gene to be copied and also signals the disconnecting of the DNA with the newly minted RNA. The initial transcripts produced from tRNA genes are precursor molecules, which are processed into mature tRNAs.
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