The Condensin Complex Is Essential for Amitotic Segregation of Bulk Chromosomes, but Not Nucleoli, in the Ciliate Tetrahymena thermophila

Abstract

The macronucleus of the binucleate ciliate Tetrahymena thermophila contains fragmented and amplified chromosomes that do not have centromeres, eliminating the possibility of mitotic nuclear division. Instead, the macronucleus divides by amitosis with random segregation of these chromosomes without detectable chromatin condensation. This amitotic division provides a special opportunity for studying the roles of mitotic proteins in segregating acentric chromatin. The Smc4 protein is a core component of the condensin complex that plays a role in chromatin condensation and has also been associated with nucleolar segregation, DNA repair, and maintenance of the chromatin scaffold. Mutants of Tetrahymena SMC4 have remarkable characteristics during amitosis. They do not form microtubules inside the macronucleus as normal cells do, and there is little or no bulk DNA segregation during cell division. Nevertheless, segregation of nucleoli to daughter cells still occurs, indicating the independence of this process and bulk DNA segregation in ciliate amitosis.

Supplemental material for this article may be found at http://mcb.asm.org/.

We are grateful to Rachel Howard-Till, Harmit Malik, Hisashi Tanaka, and the anonymous reviewers for critical review of the manuscript, which led to many improvements; to the Yao lab and Gerald Smith's lab for helpful discussions; to Nancy Schult for assistance in cloning and sequencing TtSMC4; to Peter Bruns for providing lab facilities and encouragement to R.S.C.; to Elizabeth Greene and Jonathan Badger for assistance with protein sequence and phylogenetic analysis; and to the ciliate community for support. We thank Kathy Collins, Emily Wiley, and David Allis for antibodies. Preliminary sequence data for the T. thermophila scaffolds were obtained from The Institute for Genomic Research.

This research was supported by National Institutes of Health grant GM26210 (to M.C.Y.), National Science Foundation grant MCB-0096270 (to R.S.C.), and National Institutes of Health training grant fellowship T32HD007183-26 (to M.D.C.).