Sex (Well, Not Re­ally) and the Sin­gle Or­ganelle

by Elio

We re­cently posted a piece on or­ganel­lar di­vi­sion and – for some mi­to­chon­dria – fu­sion. We em­pha­sized that in the cells of many or­gan­isms (which con­tain more than sin­gle or­ganelles) the mi­to­chon­dria lack per­ma­nent iden­tity. Rather, mi­to­chon­dria fuse with one an­other and later di­vide. Dur­ing di­vi­sion, they get pinched off by the ac­tion of spe­cial "stran­gling pro­teins," the dy­namins. Hav­ing a large copy num­ber, there may be no need for a spe­cial mech­a­nism to en­sure pre­cise seg­re­ga­tion dur­ing cell di­vi­sion. But how about or­gan­isms that have but a sin­gle mi­to­chon­drion or chloro­plast?

Cells of Cyani­dioschy­zon mero­lae (NIES‑1­332).Source

Doesn't this sim­ple fact im­ply some de­gree of strin­gency in their or­ganelle di­vi­sion? Note that this is a trick ques­tion. Be­cause for mi­to­chon­dria that fuse and break up, the num­ber of these organ­elles is in­de­ter­mi­nate. How­ever, in some uni­cel­lu­lar or­gan­isms one can rea­son­ably say that the num­ber is in­deed one of a given type of or­ganelle per cell. The ev­i­dence rests not only on mor­pho­logy but also on the co­her­ence be­tween cell di­vi­sion and or­ganelle di­vi­sion.

Cyani­dioschy­zon mero­lae cells con­tain a nu­cleus, a V‑sha­ped mi­to­chon­drion, a dumb-bell-shaped plas­tid, a mi­cro­body and a Golgi ap­pa­ra­tus, di­vi­sions of which can be highly syn­chro­nized by light/dark cy­cles. Source

A fa­vorite or­gan­ism for such stud­ies is a small red alga, Cyani­dioschy­zon mero­lae (you might as well get used to its name be­cause you'll surely hear more about it in the fu­ture). C. mero­lae has a sin­gle mi­to­chon­drion and a sin­gle chloro­plast, both of which di­vide once dur­ing each cell cy­cle, but not nec­es­sar­ily both at the same time. Be­fore go­ing into that, we should in­tro­duce the or­gan­ism. C. mero­lae is a red alga which mea­sures a mere 2 μm and lacks a rigid cell wall. It lives in acidic hot springs: pH be­low 2, tem­per­a­ture of 45° C. Con­ve­niently, di­vi­sion of the cell, the chloro­plast, and the mi­to­chon­drion can be syn­chro­nized by light/dark cy­cles. Its nu­clear and or­ganellar ge­nomes have been se­quenced, re­veal­ing se­veral unique fea­tures. For ex­am­ple, it has only three copies of ri­bo­so­mal genes, al­most no in­trons, no myosin genes, and two dy­namin genes that are used only for di­vi­sion of mi­to­chon­dria and plas­tids. At ap­prox­i­mately 16 Mbp, its nu­clear genome is among the small­est known for non-sym­bi­otic eu­kary­otes and cur­rently ranks as the small­est of all pho­to­syn­thetic eu­kary­otes. Put this all to­gether and you have a handy model or­gan­ism.

So, how do the or­ganelles di­vide in C. mero­lae ? Note that in most eu­kary­otes, or­ganelles are moved around by cy­toskele­tal el­e­ments, mi­cro­tubules and/or mi­cro­fil­a­ments. But C. mero­lae lacks a con­ven­tional cy­toskele­ton, as in­di­cated by the ab­sence of ac­to­myosin, myosin genes, and un­de­tectable ex­pres­sion of actin genes. It does, how­ever, have mi­cro­tubules and typ­i­cal α‑tubulin, as well as dy­namins (GT­Pases in­volved in mem­brane traf­fick­ing). It makes do with what it has, and uses these com­po­nents to con­nect or­ganelle di­vi­sion with mi­to­sis. The di­vid­ing mi­to­chon­drion is di­rectly as­so­ci­ated with the mi­totic spin­dle, sug­gest­ing that its di­vi­sion is phys­i­cally cou­pled with mi­to­sis and may, in­deed, be re­quired be­fore spin­dle elon­ga­tion can pro­ceed. The daugh­ter mi­tochondria are as­so­ci­ated with the spin­dle pole bod­ies and thus are seg­re­gated to the op­po­site ends of the cell. (Since this as­so­ci­a­tion also oc­curs in yeasts, the au­thors sug­gest that this mecha­nism may be wide­spread among eu­kary­otes.) As the daugh­ter mi­to­chon­dria are sev­ered, dy­namin sticks to each of the mi­to­chon­dria and to the tip of a mi­cro­tubule stretch­ing be­tween them.

Some in­ter­est­ing bits about the di­vi­sion ma­chin­ery. Di­vi­sion of both mi­to­chon­dria and chloro­plasts in­volves both FtsZ (a ho­molog of the bac­te­r­ial di­vi­sion pro­tein, now en­coded in the nu­clear genome) and dy­namin (a eu­kary­ote-spe­cific pro­tein). The iso­lated chloro­plast di­vi­sion ma­chin­ery in­cludes su­per­twisted rings and spi­rals, sug­gest­ing that these com­po­nents ac­tively con­tract dur­ing or­ganelle di­vi­sion. In­deed, the spi­rals can ac­tively con­strict af­ter be­ing stretched us­ing op­ti­cal tweez­ers.

More read­ing is re­quired to ap­pre­ci­ate fully all the de­tails. Still, you get the idea that in this eu­karyote, with the small­est of all pho­totrophic genomes, or­ganelle di­vi­sion is a re­li­able and well-reg­u­lated process co­or­di­nat­ing com­po­nents of bac­te­r­ial as well as eu­kary­otic ori­gin. When you have no du­pli­cate copies of your sin­gle or­ganelles, there is no room for er­ror.