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Proteins consist of a long sequence of amino acids and those amino acids are supplied by the so-called transfer RNA, or tRNA, molecules. The tRNA molecules, in turn, are loaded with the right amino acid by the so-called aminoacyl-transfer RNA synthetases, or aaRS, proteins. There are several different versions of aaRS proteins, which load the different tRNA molecules with the different kinds of amino acids. These aaRS proteins hang around the ribosome where proteins are constructed. But as I discussed in the previous post, the lysine aaRS, also known as LysRS, has an interesting dual role. Normally it hangs around the ribosome where it binds to another LysRS to form what is known as a dimer. In this dimer configuration, there is a weak binding between (i) the 207th amino acid (a serine) of one of the LysRS proteins and (ii) the 540th and 541st amino acids (a glycine and a leucine, respectively) of the other LysRS protein. The serine side chain consists of a hydroxyl group attached to a methyl group and when the 207th amino acid’s hydroxyl group is phosphorylated (the hydrogen is replaced by a phosphate group), its binding to the opposing 540th and 541st amino acids is broken. Not surprisingly this allows the serine dimer to loosen. But this is only the beginning of a series of events caused by the phosphorylation of that serine. The serine molecules move away from the ribosome, enter the cell’s nucleus, interact with a DNA transcription factor causing (i) the production of a signaling molecule, diadenosine tetraphosphate, (ii) release of one of the transcription factor’s proteins and (iii) expression of the transcription factor’s target genes. Simply put, depending on the phosphorylation state of its 207th amino acid, LysRS has two different structural and functional states. When not phosphorylated LysRS is a crucial part of the translation process, supplying lysine amino acids to tRNA molecules. When phosphorylated it regulates gene expression. This is a major problem for evolutionary theory. Read more