The start point is usually (90% of the time) a purine. It is common for the start point to be the central base in the sequence CAT, but the conservation of this triplet is not great enough to regard it as an obligatory signal.

TLS Online TPP Program

#Id: 7076

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 7077

#Unit 3. Fundamental Processes

ThrRS has the opposite problem: It must synthesize Thr–tRNAThr but not Val–tRNAThr. Specificity is conferred by the aminoacylation site, which contains a Zn2+ ion that is coordinated by the side chain OH group of threonine. Valine cannot coordinate the Zn2+ in this way and hence does not undergo adenylylation by ThrRS.  A separate editing site deals with misacylated Ser–tRNAThr. TyrRS distinguishes between tyrosine and phenylalanine through hydrogen missing from the repertoire are GlnRS and asparaginyl-tRNA synthetase (AsnRS). To synthesize Gln-tRNAGln and Asn-tRNAAsn, these organisms possess distinct glutamyl-tRNA synthetase (GluRS) and aspartyl-tRNA synthetase (AspRS) enzymes that are nondiscriminating (ND). GluRSND synthesizes both Glu-tRNAGlu as well as misacylated Glu-tRNAGln; AspRSND synthesizes both Asp-tRNAAsp and misacylated Asp-tRNAAsn.

TLS Online TPP Program

#Id: 7078

#Unit 3. Fundamental Processes

Selenocysteine is generated enzymatically from serine carried on a special tRNA that is charged by serine-tRNA synthetase. The Sec residues of selenoproteins are thought to participate in redox reactions such as those catalyzed by mammalian glutathione peroxidase and thioredoxin reductase

TLS Online TPP Program

#Id: 7079

#Unit 3. Fundamental Processes

Selenocysteine, sometimes called the “twenty-first amino acid,”

is incorporated into proteins with the aid of a tRNA that interprets the UGA Stop codon as a Sec codon. tRNASec is initially charged with serine in a reaction catalyzed by the same SerRS that charges tRNASer.

TLS Online TPP Program

#Id: 7080

#Unit 3. Fundamental Processes

A dedicated protein (a special elongation factor) named SELB in complex with GTP is required to deliver Sec–tRNASec to the ribosome. SELB ∙ GTP ∙ Sec–tRNASec reads the UGA codon as Sec rather than “Stop,” provided that the ribosomally bound

mRNA has a hairpin loop on the 3′ side of the UGA specifying Sec.

TLS Online TPP Program

#Id: 7081

#Unit 3. Fundamental Processes

The tRNAAla structural elements recognized by then Ala-tRNA synthetase are unusually simple.  A single G=U base pair 3:70 is the only element needed for specific binding and aminoacylation