Roles for Condensin in C. elegans Chromosome Dynamics.

Condensin complexes are essential for higher order organization of chromosome structure. Higher eukaryotes have two condensins (condensin I and II) dedicated to mitotic and meiotic chromosome dynamics. C. elegans was thought to be an anomaly, with only a single mitotic condensin (condensin II), and one specialized for dosage compensation (condensin IDC). Condensin IDC binds both hermaphrodite X chromosomes to reduce gene expression by half, equalizing X-linked gene product in males (XO) and hermaphrodites (XX), while condensin II is essential for efficient chromosome organization and segregation during mitosis and meiosis. It was proposed that the unusual holocentric chromosomes in C. elegans did not require condensin I and II to accomplish cell division, and therefore, condensin I was customized for X chromosome regulation. However, we showed that subunits from condensin IDC and condensin II interact to form a second mitotic/meiotic condensin, the bonafide C. elegans condensin I. Our findings raise C. elegans to a unique status, with three distinct condensins controlling holocentric chromosome dynamics. Condensin I and II have distinct localization patterns on mitotic and meiotic chromosomes, suggesting that their roles in chromosome organization may be distinct. During mitosis, condensin II colocalizes with the centromere while condensin I discontinuously coats chromosomes. Condensin I, but not II, colocalizes with aurora B kinase, AIR-2, and our data suggests that in mitosis, AIR-2 activity is required for the recruitment of condensin I, but not condensin II. In meiosis, condensin II localizes to the sister chromatid core, while condensin I localizes to the interface between homologous chromosomes. Condensin I and AIR-2 colocalize at this interface. Similar to mitosis, in AIR-2 depleted animals undergoing meiosis, condensin II is not affected but condensin I mislocalizes to the interface between sister chromatids, as well as homologous chromosomes. This work indicates that AIR-2 provides important spatial cues for condensin I localization on meiotic chromosomes. The contribution of condensin I to during mitosis and meiosis is not well-defined. A comparative analysis of chromosome organization and mitotic/ meiotic progression between wildtype and condensin I depleted animals will provide a better understanding of condensin I function in chromosome dynamics during cell division.Ph.D.Molecular, Cellular, and Developmental BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89697/1/karishms_1.pd

Restricting Dosage Compensation Complex Binding to the X Chromosomes by H2A.Z/HTZ-1

Dosage compensation ensures similar levels of X-linked gene products in males (XY or XO) and females (XX), despite their different numbers of X chromosomes. In mammals, flies, and worms, dosage compensation is mediated by a specialized machinery that localizes to one or both of the X chromosomes in one sex resulting in a change in gene expression from the affected X chromosome(s). In mammals and flies, dosage compensation is associated with specific histone posttranslational modifications and replacement with variant histones. Until now, no specific histone modifications or histone variants have been implicated in Caenorhabditis elegans dosage compensation. Taking a candidate approach, we have looked at specific histone modifications and variants on the C. elegans dosage compensated X chromosomes. Using RNAi-based assays, we show that reducing levels of the histone H2A variant, H2A.Z (HTZ-1 in C. elegans), leads to partial disruption of dosage compensation. By immunofluorescence, we have observed that HTZ-1 is under-represented on the dosage compensated X chromosomes, but not on the non-dosage compensated male X chromosome. We find that reduction of HTZ-1 levels by RNA interference (RNAi) and mutation results in only a very modest change in dosage compensation complex protein levels. However, in these animals, the X chromosome–specific localization of the complex is partially disrupted, with some nuclei displaying DCC localization beyond the X chromosome territory. We propose a model in which HTZ-1, directly or indirectly, serves to restrict the dosage compensation complex to the X chromosome by acting as or regulating the activity of an autosomal repellant