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DNA replication mechanisms and genome stability in
Saccharomyces cerevisiae
P a r t i c i p a n t s :
Federica Lo Sardo, graduate student.
C o l l a b o r a t i o n s :
Department of Microbiology and Molecular Genetics, UMDNJ,
New Jersey Medical School, Newark, USA (Prof. Carol S.
Newlon); Dipartimento di Scienze Biomolecolari e Biotecnologie,
Università degli Studi di Milano and Istituto FIRC di Oncologia
Molecolare, Milano (Prof. Marco Foiani); Istituto FIRC di
Oncologia Molecolare, Milano (Dr. Giordano Liberi).
Report of activity
Different biological processes are involved in the
maintenance of genome integrity as DNA replication,
repair and checkpoint mechanisms that coordinate
DNA metabolism with the progression of cell
cycle. These mechanisms have been well studied in
Saccharomyces cerevisiae and many of them are common
to higher eukaryotic organisms. Complete and
accurate DNA replication is integral to the maintenance
of the genetic integrity of all organisms.
Chromosome duplication is controlled at the level of
replication initiation, which occurs at cis-acting replicator
sequences spaced at intervals of approximately
40kb along the chromosomes of Saccharomyces cerevisiae.
Surprisingly, it has been recently shown that
derivatives of chromosome III that lack known replicators
segregated properly in at least 96% of cell
divisions. To understand the mechanisms that maintain
these “originless” chromosome fragments,
mutants defective in the maintenance of an “originless”
chromosome fragment were isolated (originless
fragment maintenance mutants). Only three of them,
OFM1, ofm6 and ofm14, meet the ofm criterion
because they are defective only in the maintenance of
the “originless” fragment but proficient in the maintenance
of the same fragment that carries its normal
complement of replicators. The study of ofm mutants
can help to define the mechanisms responsible of the
Principal investigator: Lucia Fabiani
Researcher in Molecular Biology
Dipartimento di Biologia Cellulare e dello Sviluppo
Tel: 06 49912243; Fax: (+39) 06 49912351
lucia.fabiani@uniroma1.it
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Molecular genetics of eukaryotes - AREA 3
replication of the “originless” fragment and a more
detailed analysis of ofm2 and ofm5 mutants will allow
us to understand also the mechanisms and the genes
involved in chromosome stability both being mutants
characterized by a high loss rate of the wildtype
derivative of chromosome III. We have recently identified
the gene able to complement the ofm5 mutation.
CHL1 (Chromosome Loss 1) is the mutated gene.
Chl1p (861 aa) is a very interesting protein, member
of DEAH helicases family-ATP dependent, its helicase
activity is important for the establishment of sister
chromatid cohesion (SCC) during S phase but its
role is poorly understood. Chl1p interacts physically
and genetically with Eco1p (Establishment cohesion
1) involved in the regulation of SCC establishment.
The sister chromatid cohesion is an essential mechanism
for proper chromosome segregation and DNA
double strand breaks repair by homologous recombination.
The SSC required the coordinated activites of
several factors: cohesins, deposition and cohesion
establishment proteins. Mutations in any of these factors
result in precocious sister separation, aneuploidy
and cell death. Mutations in human cohesion factors
are known to contribute to cancer progression and
premature aging. Recently it has been shown (A) a
correlation between DNA replication and sister chromatid
cohesion, proteins important for DNA replication
interact directly with proteins involved in sister
chromatid cohesion, and (B) the establishment of
SCC is not restricted to S phase but occurs also after
DNA damage.
Our interest is to understand the role of Chl1p in the
establishment of sister chromatid cohesion during
DNA replication in S. cerevisiae as a model system.
Chl1p shares a high similarity with human BACH1
(BRCA1 Associated C-terminal Helicase) protein
that directly interact with the tumor suppressor
BRCA1 (BReast CAncer 1); mutations in BACH1 are
associated to breast cancer and genetically linked to
the bone marrow disease Fanconi Anemia (FA).
Chl1p, in normal condition, could be recruited at the