Phosphorylation of the SQ H2A.X Motif Is Required for Proper Meiosis and Mitosis in Tetrahymena thermophila

ABSTRACT Phosphorylation of the C terminus SQ motif that defines H2A.X variants is required for efficient DNA double-strand break (DSB) repair in diverse organisms but has not been studied in ciliated protozoa. Tetrahymena H2A.X is one of two similarly expressed major H2As, thereby differing both from mammals, where H2A.X is a quantitatively minor component, and from Saccharomyces cerevisiae where it is the only type of major H2A. Tetrahymena H2A.X is phosphorylated in the SQ motif in both the mitotic micronucleus and the amitotic macronucleus in response to DSBs induced by chemical agents and in the micronucleus during prophase of meiosis, which occurs in the absence of a synaptonemal complex. H2A.X is phosphorylated when programmed DNA rearrangements occur in developing macronuclei, as for immunoglobulin gene rearrangements in mammals, but not during the DNA fragmentation that accompanies breakdown of the parental macronucleus during conjugation, correcting the previous interpretation that this process is apoptosis-like. Using strains containing a mutated (S134A) SQ motif, we demonstrate that phosphorylation of this motif is important for Tetrahymena cells to recover from exogenous DNA damage and is required for normal micronuclear meiosis and mitosis and, to a lesser extent, for normal amitotic macronuclear division; its absence, while not lethal, leads to the accumulation of DSBs in both micro- and macronuclei. These results demonstrate multiple roles of H2A.X phosphorylation in maintaining genomic integrity in different phases of the Tetrahymena life cycle.

The CNA1 Histone of the Ciliate Tetrahymena thermophila Is Essential for Chromosome Segregation in the Germline Micronucleus

Ciliated protozoans present several features of chromosome segregation that are unique among eukaryotes, including their maintenance of two nuclei: a germline micronucleus, which undergoes conventional mitosis and meiosis, and a somatic macronucleus that divides by an amitotic process. To study ciliate chromosome segregation, we have identified the centromeric histone gene in the Tetrahymena thermophila genome (CNA1). CNA1p specifically localizes to peripheral centromeres in the micronucleus but is absent in the macronucleus during vegetative growth. During meiotic prophase of the micronucleus, when chromosomes are stretched to twice the length of the cell, CNA1p is found localized in punctate spots throughout the length of the chromosomes. As conjugation proceeds, CNA1p appears initially diffuse, but quickly reverts to discrete dots in those nuclei destined to become micronuclei, whereas it remains diffuse and is gradually lost in developing macronuclei. In progeny of germline CNA1 knockouts, we see no defects in macronuclear division or viability of the progeny cells immediately following the knockout. However, within a few divisions, progeny show abnormal mitotic segregation of their micronucleus, with most cells eventually losing their micronucleus entirely. This study reveals a strong dependence of the germline micronucleus on centromeric histones for proper chromosome segregation.

A β-Tubulin Mutation Selectively Uncouples Nuclear Division and Cytokinesis in Tetrahymena thermophila

ABSTRACT The ciliated protozoan Tetrahymena thermophila contains two distinct nuclei within a single cell—the mitotic micronucleus and the amitotic macronucleus. Although microtubules are required for proper division of both nuclei, macronuclear chromosomes lack centromeres and the role of microtubules in macronuclear division has not been established. Here we describe nuclear division defects in cells expressing a mutant β-tubulin allele that confers hypersensitivity to the microtubule-stabilizing drug paclitaxel. Macronuclear division is profoundly affected by the btu1-1 (K350M) mutation, producing cells with widely variable DNA contents, including cells that lack macronuclei entirely. Protein expressed by the btu1-1 allele is dominant over wild-type protein expressed by the BTU2 locus. Normal macronuclear division is restored when the btu1-1 allele is inactivated by targeted disruption or expressed as a truncated protein. Immunofluorescence studies reveal elongated microtubular structures that surround macronuclei that fail to migrate to the cleavage furrows. In contrast, other cytoplasmic microtubule-dependent processes, such as cytokinesis, cortical patterning, and oral apparatus assembly, appear to be unaffected in the mutant. Micronuclear division is also perturbed in the K350M mutant, producing nuclei with elongated early-anaphase spindle configurations that persist well after the initiation of cytokinesis. The K350M mutation affects tubulin dynamics, as the macronuclear division defect is exacerbated by three treatments that promote microtubule polymerization: (i) elevated temperatures, (ii) sublethal concentrations of paclitaxel, and (iii) high concentrations of dimethyl sulfoxide. Inhibition of phosphatidylinositol 3-kinase (PI 3-kinase) with 3-methyladenine or wortmannin also induces amacronucleate cell formation in a btu1-1-dependent manner. Conversely, the myosin light chain kinase inhibitor ML-7 has no effect on nuclear division in the btu1-1 mutant strain. These findings provide new insights into microtubule dynamics and link the evolutionarily conserved PI 3-kinase signaling pathway to nuclear migration and/or division in Tetrahymena.

Spatiotemporal control of functional specification and distribution of spindle microtubules with 13, 14 and 15 protofilaments during mitosis in the ciliate Nyctotherus

The formation of microtubules with more than 13 protofilaments in the ciliate Nyctotherus ovalis Leidy seems to be a highly ordered process. Such microtubules are restricted to the nucleoplasm and, moreover, to certain stages of nuclear division. They assemble during anaphase of micronuclear mitosis and during the elongation phase of macronuclear division. The number of microtubules with more than 13 protofilaments in the micronuclear nucleoplasm increases as anaphase progresses. Furthermore, assembly of microtubules with 14 and 15 protofilaments seems to proceed concomitantly with net disassembly of 13-protofilament microtubules, because the total amount of polymerized tubulin in the interpolar spindle region remains approximately constant between mid anaphase and late telophase. In addition, evidence for spatial control of the distribution of microtubules with different protofilament numbers in the micronuclear stembody has been found. The percentage of microtubules with 13 protofilaments per stembody cross-section is highest at the ends of the stembody, while the percentage of microtubules with either 14 or 15 protofilaments increases as the middle of the stembody is approached. Temporal control of polymerization of microtubules with high protofilament numbers seems to be exerted independently in the two types of nuclei. For example, when the macronucleus starts to elongate it contains microtubules with more than 13 protofilaments but the metaphase micronucleus still possesses only microtubules with 13 protofilaments at this stage. Control of fidelity of protofilament numbers is not lost in the early stages of micronuclear or macronuclear division when cells are exposed to 2H2O or media containing taxol. Even microtubules that reassemble during recovery of metaphase micronuclei from nocodazole-induced microtubule depolymerization, in either the absence or presence of 2H2O and taxol, possess 13 protofilaments. Similarly, if the introduction of microtubules with 14 and 15 protofilaments is inhibited during early micronuclear anaphase and delayed for 60 min by exposure to nocodazole, such microtubules still assemble during telophase when recovery is permitted. Microtubules that have been assembled under normal conditions show differential sensitivity to nocodazole. During metaphase, nocodazole induces disassembly of most microtubules. There is an increase in microtubule stability that coincides with the appearance of microtubules with high protofilament numbers during early anaphase. However, considerable numbers of 13-protofilament microtubules, as well as microtubules with 14 and 15 protofilaments, exhibit such stability during anaphase.(ABSTRACT TRUNCATED AT 400 WORDS)

Evolutionary constraints on quantitative variation and regulation of macronuclear dna content in the genus Tetrahymena

Fluorescence cytophotometry was used to examine quantitative variation and regulation of macronuclear DNA content in 44 clones representing 13 species in 3 species-complexes of the genus Tetrahymena. Mean DNA amounts for G2 macronuclei generally ranged from 20–50 pg, with extreme means of 10 and 196 pg. Both intra- and interspecific variation was usually significant at the 5% level, and in some instances DNA amounts for the same clone in repeated experiments were significant different. Nevertheless, intraclonal ranges both within and among species frequently showed considerable overlap and, in the context of all the ciliates, the range of means is small. Calculations suggest that a biologically more meaningful measure, gene dosage, is also evolutionarily conserved. Additional evolutionary constraints are found in the regulation of macronuclear DNA content. Analysis of G1 and G2 intraclonal variances shows that in all species the variance added by regular unequal macronuclear division is removed by modification of cell-cycle events according to Model II regulation. In Model II, macronuclei with a small amount of DNA undergo an additional S phase before nuclear and cell division, whereas macronuclei with a large amount of DNA omit an S phase. Chromatin extrusion is also a regular feature of macronuclear division in most species, but its role in regulation is unclear. Extrusion regulates downward the mean amount of DNA but may actually contribute to unequal division and therefore add rather than remove variance.

Microtubules and control of macronuclear ‘amitosis’ in Paramecium

The ‘amitotic’ division of the macronucleus during binary fission in P. tetraurelia includes a detailed sequence of shape changes that are temporally coordinated with the adoption of a series of well-defined positions and orientations inside the cell. The deployment of nucleoplasmic microtubules that is spatially correlated with the shaping ritual is more complex and precise than has been reported previously. Macronuclear division is not amitotic. It is not a simple constriction into two halves. As a dividing macronucleus starts to elongate it becomes dorsoventrally flattened against the dorsal cortex of the organism and assumes an elliptical shape. Concurrently, an elliptical marginal band of intranuclear microtubules assembles that has the same spatial relationship to nuclear shape as the marginal microtubules assembles that has the same spatial relationship to nuclear shape as the marginal microtubule bands of certain elliptical vertebrate blood cells have to cell shape. The band breaks down as further elongation occurs and the nucleus adopts the shape of a straight and slender sausage. Most of the intranuclear microtubules assemble as elongation starts and break down shortly after elongation is completed; the majority are oriented parallel to the longitudinal axis of the nucleus throughout elongation. Some of them are attached to nucleoli and are coated with granules which are almost certainly derived from the cortices of nucleoli. The peripheral concentration, interconnexion, orientation, and overlapping arrangement of microtubules, and the reduction in microtubule number per nuclear cross-section as elongation proceeds at a rate of about 40 micrometers min-1, are all compatible with the provision of a microtubule sliding mechanism as the main skeletal basis for elongation. There are indications that this mechanism is augmented by anchorage and/or active propulsion of nucleoli that may perhaps facilitate fairly equitable segregation of chromosomal material to daughter nuclei.

Abnormal microtubule deployment during defective macronuclear division in a Paramecium mutant

The tam 8 mutant of Paramecium tetraurelia is a representative of a class of mutants characterized by abnormal nuclear divisions during binary fission and the failure of trichocysts to attach to the plasma membrane. Compared with wild-type organisms the following abnormalities occur in tam 8 individuals. (I) The spherical interphase macronucleus is not positioned near the oral apparatus; it is randomly located in the cytoplasm of interfission organisms. (2) The macronucleus does not migrate towards the anterior dorsal cortex as its division starts, nor is it dorsally and subcortically positioned as it elongates. (3) Elongating macronuclei exhibit variable and irregular shapes. (4) This elongation is delayed and reduced. (5) Longitudinally oriented microtubules assemble in the nucleoplasm of dividing macronuclei but their spatial deployment is abnormal. (6) Unequal segregation of micronuclei between daughter organisms occurs during binary fission. The abnormal arrangement of nucleoplasmic microtubules provides support for the proposal that a microtubule sliding mechanism is involved during the elongation of dividing macronuclei. The extent to which macronuclear division may be controlled by the cell cortex is considered in relation to tthe pleiotropic effects of the tam 8 mutation.