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18.2 SAM-Dependent Methyltransferases  405
























                                     NH 2
                                                             NH 2
                                    N                       N     OH
                                  O  N     C5 MTase       O   N
                                           + HCHO
                            DNAO                    DNAO
                                   O       − HCHO          O
                                 ODNA                    ODNA
                            (a)    C                       hmC

                       NH 2                    NH 2                      NH 2
                                                     Linker-NH 2               Linker-NH
                     N      OH               N      S                  N      S
                    O  N                   O   N                     O   N
             DNAO                    DNAO                      DNAO
                    O                       O                         O
                   ODNA                    ODNA                     ODNA
             (b)    hmC
             Scheme 18.8 (a) Reversible conversion of cytosine (C) into hydroxymethylcytosine (hmC) with formaldehyde and methyltrans-
             ferases [53a]. (b) Methyltransferase-dependent derivatization of hmC with either cysteamine or selenocysteamine and further func-
             tionalization with any fluorescent group [53b].
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