Orderly formation of the double ring structures for plastid and mitochondrial division in the unicellular red alga Cyanidioschyzon merolae |
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Authors: | Shin-ya Miyagishima Ryuuichi Itoh Kyoko Toda Hidenori Takahashi Haruko Kuroiwa Tsuneyoshi Kuroiwa |
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Institution: | (1) Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Tokyo 113-0033, Japan, JP;(2) Kyoritsu Women's Junior College, Kanda, Tokyo 101-0051, Japan, JP |
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Abstract: | The formation of the plastid-dividing ring (PD ring) and mitochondrion-dividing ring (MD ring) was studied in a highly synchronous
culture of the unicellular red alga Cyanidioschyzon merolae. The timing and the order of formation of the MD and PD rings were determined by observing organelles around the onset of
their division, using transmission electron microscopy. In C. merolae, there is one chloroplast and one mitochondrion per cell, and the shape of the chloroplast changes sequentially from acorn-like,
to round, to trapezoidal, to peanut-shaped, in that order, during the early stage of chloroplast division. None of the cells
with acorn-shaped or round chloroplasts contained organelles with PD rings or MD rings, while all of the cells with peanut-shaped
chloroplasts contained organelles with both PD rings and MD rings. In cells with peanut-shaped chloroplasts, the PD and MD
rings were double ring structures, with an outer ring located on the cytoplasmic face of the outer membrane of the organelle,
and an inner ring located in the matrix beneath the inner membrane. These results suggested that the double ring structures
of the PD ring and the MD ring form when chloroplasts are trapezoidal in shape. Detailed three-dimensional observation of
cells with trapezoidal chloroplasts revealed the following steps in the formation of the double ring structures of the PD
and MD rings: (i) the inner ring of the PD ring forms first, followed by the outer ring; (ii) then the MD ring forms and becomes
visible; (iii) when the double ring structures of the two rings have formed, the microbody then moves from its remote location
to the plane of division of the mitochondrion and contraction of the PD and MD rings commences. These steps were also confirmed
by computer-aided three-dimensional reconstruction of the images from serial thin sections. This study reveals the order of
formation of the double ring structures of the PD and MD rings, and the behavior of the microbody around the onset of division
of plastids and mitochondria. The results also provide the first evidence that the inner PD ring is not a tension element
formed by the contractile pressure but a definite structure, independent of the outer ring.
Received: 31 March 1998 / Accepted: 14 May 1998 |
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Keywords: | Key words:Cyanidioschyzon Microbody Mitochondrion-dividing ring Organelle division Plastid-dividing ring |
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