Monokaryotic chloroplast 1 protein (MOC1) and similar proteins
Monokaryotic chloroplast 1 protein (MOC1) is a Holliday junction (HJ) resolvase, originally identified from the green algae Chlamydomonas reinhardtii, but was later shown to be genetically conserved among green plants, such as Arabidopsis thaliana and Zea mays. Nicotiana tabacum MOC1 (NtMOC1) is exclusively localized to chloroplasts, and is essential for chloroplast nucleoid segregation. Holliday junction resolvases (HJRs) are endonucleases that specifically resolve Holliday junction DNA intermediates during homologous recombination. NtMOC1 cleaves only four-way junctions and specifically introduces nicks between two cytosines at the homology core. NtMOC1 has the highest structural similarity to RuvC resolvase, suggesting their potentially similar HJ binding and resolution activities. However, the molecular basis for specific HJ resolution by MOC1 remains elusive. This family also includes bacterial homologs.
Comment:Nicotiana tabacum chloroplast resolvase MOC1 (6KVO) contains four acidic amino acids, namely D116, D118, E175, and E258, which coordinate a magnesium ion approach to the cleavable phosphodiester backbone, constituting the catalytic tetrad.
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