FIG. 2.

(A) Structures of aspartic acid and the modified β-methylthio-aspartic acid. (B) Amino acid alignment of the P90 loop, showing the conservation of D88 (in boldface) throughout three domains of life. Bacteria are represented by T. thermophilus, E. coli, and B. subtilis; chloroplast (chl) S12 sequences are shown for Arabidopsis thaliana and Chlamydomonas reinhardtii; mitochondrial (mito) S12 sequences are shown for Saccharomyces cerevisiae and Drosophila melanogaster; and eukaryotic nuclear (nuc) S12 sequences are represented by S. cerevisiae and Homo sapiens. A protein-protein BLAST search with T. thermophilus S12 returned 300 nonredundant bacterial S12 proteins. Chloroplast, mitochondrial, and nuclear S12 sequences were retrieved from 345 eukaryotic matches. Thirty-four sequences were exhaustive of the archaeal S12 proteins retrieved, and here they are represented by Sulfolobus solfataricus, Methanococcus jannaschii, and Haloarcula marismortui.