Alladi Ramakrishnan Hall

Determinants of a rapidly evolving chromosomal locus - the centromere

Kaustuv Sanyal

JNCASR Bengaluru

Chromosome segregation is a high-fidelity process that requires timely and dynamic
interactions between the centromere-kinetochore complex and the spindle microtubules.
More than 100 proteins assemble on centromere DNA to form the kinetochore - the
chromosomal attachment site of the spindle apparatus. A specialized centromere-specific
histone, CENP-A, binds to centromere DNA to nucleate kinetochore formation in most
eukaryotes. The process of centromere/kinetochore formation is intriguing since it helps
cells to propagate critical information from the previous generation to reuse it for future
episodes of chromosome segregation. Using several human and plant pathogenic fungal
species as models to study centromere evolution, we discovered that centromeres show an
unsually high level of structural and DNA sequence diversity. Comparative genomic
analyses suggest that centromeres are also involved in chromosomal rearragements and
karyotype evolution that might have led to speciation. While active centromeres, that are
marked by CENP-A chromatin, are subject to position effects, some drifts around the site of
centromere assembly is tolerated making it a malleable genetic locus. In case a native
centromere is inactivated, occasionally neocentromeres come to the rescue. However, the
site of neocentromere formation is elusive in organisms with centromeres that do not rely on
the primary DNA sequence. It is also known that replication origins flanking centromeres not
only advance the replication timing of centromeres but also enable in loading new CENP-A
molecules in the budding yeast Candida albicans, whose centromeres exhibit epigenetic
regulation. Hence, there is an intimate connection between the replication machinery and its
role in centromere specification. We show that the pre-replication complex protein Orc4 is
highly enriched at all centromeres in this organism marking it an essential component of
functional centromeres. The genetic underpinning of this association will be discussed.