MACRONUCLEAR DNA SYNTHESIS IN Stentor : REGULATION BY A CYTOPLASMIC INITIATOR

Abstract

The large ciliate Stentor coeruleus lends itself well to microsurgical procedures. By combining these techniques with autoradiography after exposure of cells to thymidine-H3, it is possible to study mechanisms regulating the occurrence of macronuclear DNA synthesis during the cell growth cycle. Transfer of nuclei between cells which are synthesizing DNA and cells which are not can indicate whether DNA synthesis is determined by the continuous presence or absence of a cytoplasmic factor. Similar experiments involving cell grafting could characterize such a factor as an initiator or an inhibitor. This paper describes these experiments and their results.Previous work on Stentor has suggested that mechanisms regulating the occurrence of several major events in the cell growth cycle are located in the cytoplasm.3-5The present results will be discussed in terms of these earlier findings and also in relation to current knowledge about regulation of nuclear DNA synthesis.Stentor coeruleus is a large heterotrich ciliate; individual organisms often reach a length of 1 mm when fully extended. Figure 1A shows the main morphological features of S. coeruleus, including the chain macronucleus, the anteriorly placed oral apparatus, and the rows of blue-green pigment granules running longitudinally between the kineties.Throughout interphase, the macronucleus exists as a chain of nodes spiralling almost the entire length of the organism and situated directly beneath the ectoplasm. At 20'C, amitotic division takes approximately eight hours. Tartar has assigned numbers to the stages of amitotic division.25 There are eight stage numbers, each covering a successive one-hour interval so that stage 1 designates the first hour of division while stage 8 refers to the eighth hour, during which cytokinesis occurs. The macronucleus undergoes a series of striking morphological changes during amitotic division. At stage 5, the nuclear nodes begin to coalesce until the nucleus is a compact mass in the center of the cell (stage 6 ; Fig. 1B

The Macronuclear Genome of Stentor coeruleus Reveals Tiny Introns in a Giant Cell

“…Scaling of ploidy with cell size agrees with observations that macronuclear DNA synthesis occurs throughout interphase in Stentor [38], and suggests DNA content may determine cell size in Stentor or vice versa.…”

Detection of G1 proteins in Chinese hamster cells synchronized by isoleucine deprivation or mitotic selection.

“…Kolodny and Gross (9) also reported differences in patterns of proteins synthesized during G2 phase and patterns obtained in other phases of the cell cycle. Numerous studies suggest that factors required for initiation of DNA synthesis are made in the cytoplasm (3,6,7,17,18, and others). Our results showed that the peak I protein was synthesized in the cytoplasm of cells traversing G1 and that synthesis declined as the cells entered S phase.…”

The Chromosome Cycle

“…By culturing dividing organisms (stages 3-8) in a 3H-TdR containing medium for 30 min and then making autoradiographs of the organisms, DE TERRA (1967) found that label was always present above the nuclear nodes at stages 3 and 4, seldom present above coalescing nuclei (stage 5) and never present above the compacted, elongating and nodulating nuclei (stages 6-8). DNA synthesis thus occurs during the first four hours of the 8 hour division period (D) and stops at or near the time nuclear coalescence begins.…”