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Kyoto University researchers uncover 'DNA clip' mechanism that bundles chloroplast DNA

This combined image shows fluorescence microscopic images of a chloroplast. The top image shows DNA folded within the cell, as indicated by the yellow light, while HBD1 protein is functioning. The bottom image shows the DNA scattered, as indicated by the yellow light, as the HBD1 protein lost its function due to genome editing. (Photo courtesy of Yoshiki Nishimura, assistant professor at Kyoto University's Graduate School of Science)

KYOTO -- A team including Kyoto University researchers has revealed the unique workings of a "DNA clip" found in algae and other plants that folds and bundles the DNA of chloroplasts used in photosynthesis so that they fit compactly within cells.

    The team said that protein which has what resembles "two hands" folds as well as bundles up and binds the DNA. The findings were published May 10 in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

    The team, consisting of Yoshiki Nishimura, assistant professor specializing in plant molecular genetics at Kyoto University's Graduate School of Science, researcher Mari Takusagawa and others, used the green alga of Chlamydomonas in its research. It measured some 100 protein clumps contained in nucleoids packed with chloroplast DNA, using mass spectrometry, and discovered HBD1, a protein that attaches itself to DNA in two places. The team uncovered that if the genes making up this protein are disrupted through genome editing, the DNA becomes undone and scatters in small pieces.

    In nucleoids, DNA exists as two long strands which are tied up in a bundle at a length of around 1/3000 the original length, and are a size of about 0.2 micrometers. The team also noted that even if the HBD1 protein is not functioning, the cell's activities do not break down, and it is still capable of photosynthesis.

    Assistant professor Nishimura said, "It's astonishing that the plant can still photosynthesize even if the DNA becomes unraveled. It is thought that the structure of nucleoids is related to the replication and repair of DNA, as well as gene expression and genetic inheritance among other factors, and we'd like to elucidate these matters in future research."

    (Japanese original by Norikazu Chiba, Kyoto Bureau)

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