Cohesins and chromosomal cohesion in the model ciliate Tetrahymena

The cohesion of sister chromatids in the interval between chromosome replication and anaphase is important both for preventing the precocious separation and hence nondisjunction of chromatids and for facilitating DNA damage repair from the sister. Cohesion is accomplished by a ring-shaped protein complex, cohesin, and its release at anaphase is caused by the cleavage of the complex`s kleisin subunit. A popular model of cohesion is that sister chromatids are encircled by cohesin rings and separate upon opening of the rings. During meiosis, cohesion, together with chiasmata, has the additional function of holding bivalents together. Accordingly, release from cohesion in meiosis takes place in two steps: First, loss of arm cohesion allows the separation of homologous chromosomes, and second, loss of cohesion around the centromeres allows the disjunction of sister chromatids at anaphase II. This stepwise release is accomplished by a meiosis-specific version of cohesin whose kleisin is protected from cleavage around the centromeres during anaphase I. Over the recent years, evidence has been accumulating for additional roles of cohesin in regulating gene activity. Due to the cohesin`s roles in chromosome segregation, DNA damage repair and gene regulation, defects in the cohesin network cause serious developmental disorders and are the prime cause of stillbirths and congenital diseases in humans. The essence of cohesin activity and its wide conservation have been confirmed by studies primarily in fungal and animal, and to a lesser extent, plant model systems. Here, we want to put to the test the generality of our present view of cohesin and cohesion by studying the corresponding structures and functions in the evolutionarily distant protist Tetrahymena. Preliminary observations by our lab have suggested some remarkable deviations from the canonical cohesion system, such as the existence of a single version of cohesin for the use in both mitosis and meiosis. Also, cohesin does not seem to detach from chromosome arms in anaphase. Not least, Tetrahymena has mitosis/meiosis and transcription performed by different nuclei within one and the same cell. This offers the unique possibility to experimentally separate the functions of cohesin in governing chromosome segregation and gene regulation.

The cohesion of sister chromatids in the interval between chromosome replication and anaphase is important both for preventing the precocious separation and hence nondisjunction of chromatids and for facilitating DNA damage repair from the sister. Cohesion is accomplished by a ring-shaped protein complex, cohesin, and its release at anaphase is caused by the cleavage of the complexs kleisin subunit. A popular model of cohesion is that sister chromatids are encircled by cohesin rings and separate upon opening of the rings. During meiosis, cohesion, together with chiasmata, has the additional function of holding bivalents together. Accordingly, release from cohesion in meiosis takes place in two steps: First, loss of arm cohesion allows the separation of homologous chromosomes, and second, loss of cohesion around the centromeres allows the disjunction of sister chromatids at anaphase II. This stepwise release is accomplished by a meiosis-specific version of cohesin whose kleisin is protected from cleavage around the centromeres during anaphase I. Over the recent years, evidence has been accumulating for additional roles of cohesin in regulating gene activity.Due to the cohesins roles in chromosome segregation, DNA damage repair and gene regulation, defects in the cohesin network cause serious developmental disorders and are the prime cause of stillbirths and congenital diseases in humans.The essence of cohesin activity and its wide conservation have been confirmed by studies primarily in fungal and animal, and to a lesser extent, plant model systems. Here, we tested the generality of our present view of cohesin and cohesion by studying the corresponding structures and functions in the evolutionarily distant protist Tetrahymena. We found some remarkable deviations from the canonical cohesion system, such as the existence of a single version of cohesin for the use in both mitosis and meiosis. Also, cohesin does not seem to detach from chromosome arms in anaphase. Not least, we have demonstrated the particular suitability of the Tetrahymena model system for studying cohesin: It has mitosis/meiosis and transcription performed by different nuclei within one and the same cell. This offered the unique possibility to experimentally separate the functions of cohesin in governing chromosome segregation and gene regulation.