<P> Certain organisms can have one or more aminoacyl tRNA synthetases missing . This leads to charging of the tRNA by a chemically related amino acid, and by use of a catalyst enzyme or enzymes, the tRNA is modified to be correctly charged . For example, Helicobacter pylori has glutaminyl tRNA synthetase missing . Thus, glutamate tRNA synthetase charges tRNA - glutamine (tRNA - Gln) with glutamate . An amidotransferase then converts the acid side chain of the glutamate to the amide, forming the correctly charged gln - tRNA - Gln . </P> <P> The ribosome has three binding sites for tRNA molecules that span the space between the two ribosomal subunits: the A (aminoacyl), P (peptidyl), and E (exit) sites . In addition, the ribosome has two other sites for tRNA binding that are used during mRNA decoding or during the initiation of protein synthesis . These are the T site (named elongation factor Tu) and I site (initiation). By convention, the tRNA binding sites are denoted with the site on the small ribosomal subunit listed first and the site on the large ribosomal subunit listed second . For example, the A site is often written A / A, the P site, P / P, and the E site, E / E. The binding proteins like L27, L2, L14, L15, L16 at the A - and P - sites have been determined by affinity labeling by A.P. Czernilofsky et al. (Proc. Natl. Acad. Sci, USA, pp 230--234, 1974). </P> <P> Once translation initiation is complete, the first aminoacyl tRNA is located in the P / P site, ready for the elongation cycle described below . During translation elongation, tRNA first binds to the ribosome as part of a complex with elongation factor Tu (EF - Tu) or its eukaryotic (eEF - 1) or archaeal counterpart . This initial tRNA binding site is called the A / T site . In the A / T site, the A-site half resides in the small ribosomal subunit where the mRNA decoding site is located . The mRNA decoding site is where the mRNA codon is read out during translation . The T - site half resides mainly on the large ribosomal subunit where EF - Tu or eEF - 1 interacts with the ribosome . Once mRNA decoding is complete, the aminoacyl - tRNA is bound in the A / A site and is ready for the next peptide bond to be formed to its attached amino acid . The peptidyl - tRNA, which transfers the growing polypeptide to the aminoacyl - tRNA bound in the A / A site, is bound in the P / P site . Once the peptide bond is formed, the tRNA in the P / P site is deacylated, or has a free 3' end, and the tRNA in the A / A site carries the growing polypeptide chain . To allow for the next elongation cycle, the tRNAs then move through hybrid A / P and P / E binding sites, before completing the cycle and residing in the P / P and E / E sites . Once the A / A and P / P tRNAs have moved to the P / P and E / E sites, the mRNA has also moved over by one codon and the A / T site is vacant, ready for the next round of mRNA decoding . The tRNA bound in the E / E site then leaves the ribosome . </P> <P> The P / I site is actually the first to bind to aminoacyl tRNA, which is delivered by an initiation factor called IF2 in bacteria . However, the existence of the P / I site in eukaryotic or archaeal ribosomes has not yet been confirmed . The P - site protein L27 has been determined by affinity labeling by E. Collatz and A.P. Czernilofsky (FEBS Lett., Vol. 63, pp 283--286, 1976). </P>

Give two functions that trna has that assist in translation