CYTOKINESIS BY A CONTRACTILE RING IN THE PRIMITIVE RED ALGA CYANIDIUM-CALDARIUM RK-1

To better understand the mechanism of cytokinesis is eukaryotes, the b ehavior of the contractile ring in the two unicellular primitive red a lgae Cyanidioschyzon merolae and Cyanidium caldarium RK-1, which have the smallest genome size among eukaryotes, was examined by fluorescein isothiocyanate (FITC)-phalloidin fluorescence microscopy, fluorometry using a video-intensified microscope photon-counting system, transmis sion electron microscopy and immunoblotting techniques. Cells in each alga contained one nucleus, one mitochondrion and one chloroplast, whi ch were aligned in that order. During cytokinesis in C. merolae, a con tractile ring was not observed by fluorescence microscopy or by transm ission electron microscopy. In contrast, in C. caldarium RK-1, a contr actile ring appeared at the equatorial region of the dividing cells an d began to contract from the side of the chloroplast. During contracti on of this ring, the total fluorescent intensifies due to FITC-phalloi din remained constant. Electron microscopy revealed outer and inner ba nds approximately 80 mm wide and 9 nm thick which ran parallel to each other just beneath the cell membrane. These bands were visible at the equator of the cell just before the initiation of cytokinesis and con stricted from the pole of the chloroplast. Both bands increased in wid th as cleavage progressed. The inner ring consisted of a bundle of app roximately 20 actin-like filaments which were arranged as a raft. In t he outer ring, such fine filaments were not visible. It seems likely t hat the bundle of filaments, known as the contractile ring, is compose d of two different elements: an inner band of actin filaments and an o uter band of unknown materials. Western immunoblotting with anti-actin antibody gave negative results in C. merolae, but was positive in C. caldarium RK-1. These results indicate that the contractile ring in C. caldarium RK-I consists of a constant amount of actin filaments throu ghout cytokinesis and that cleavage is caused by the contractile ring. Furthermore, these results suggest that C. merolae may use markedly d ifferent actin than C. caldarium RK-1 in contractile mechanisms.