Transcript Slide 1
Raghav Ramachandran
Ambhi Ganesan
September 9,2008
Test out various hypotheses
on a smaller scale.
Saves time and money.
Practically impossible to
carry out certain research
directly on intended targets.
Learn from mistakes and go
back to the drawing board.
Insights into the complex
mechanisms of Life, that are
conserved over different
levels.
Neurodegenerative disorders:
• Ease of expt’al manipulation, high conservation of mechanisms,
well defined genome
• Studying mechanisms in yeast provide insights into human
system.
• Designing therapeutics/drugs against homologous protein
aggregates in yeast.
Aging:
• Calorie restriction affects life span in worms, flies, and rodents,
apart from yeast (reducing glucose or amino acid concns can
increase life spans)
• Life span extension from Sir2-overexpression, TOR-inhibition,
Sch9/Akt or DR has been observed in yeast, worms, and flies
• Nutrient responsive protein TOR speculated to play conserved
role.
The
S.cerevisiae yeast has one of the
smallest genomes of eukaryotes, being
unicellular.
Its genome contains 12, 495,682 base
pairs and 5770 genes as opposed to 3.3
*109 base pairs and ~20500 genes in
humans.
Also, S.cerevisiae (baker’s yeast) is viable
with numerous markers and available in
large quantities making it cheap to study.
Perhaps, the
most striking feature of
S.cerevisiae is its existence in both
haploid and diploid forms.
This makes it easy to isolate recessive
mutations in haploids.
Also, DNA transformed in S.cerevisiae
can undergo homologous recombination
readily, into the S.cerevisiae genome.
By
analyzing the S.cerevisiae mutants
observed from homologous
recombination of foreign DNA, the
functions of several proteins in vivo can
be discerned.
The entire genome of S.cerevisiae was
sequenced in 1996 and since then, it has
been used as a eukaryotic model for the
study of protein interactions and
infectious diseases.
Each Yeast
Cell undergoes four phases in
its life cycle: G1,S,G2,M (growth,
synthesis, mitosis)
In S. cerevisiae, arrangement of
microtubules and duplication of spindle
pole bodies takes place early in the life
cycle to allow for bud formation.
Thus, budding S.cerevisiae lacks clear
distinction between S, G2 and M phases.
In
the G1 phase of the cycle, the yeast
cell has three options
• It can complete the cycle and divide
• It can leave the cycle, if nutritionally starved,
where it is resistant to heat and chemical
treatment
• It can mate with a cell of opposite sex, if haploid,
after a transient arrest in G1.
• It can undergo meiosis to produce four haploid
cells under nutritional starvation, if diploid
A
gene is a cell cycle regulator if its
mutant causes inappropriate progression
through the cycle.
CDC 28-p34 protein kinase in G1/S and
G2/M
CLN1, CLN2, WHI1-G1 cyclins in G1/S
MIH1- inducer of mitosis in G2/M
CKS 1, CDC 37, CDC 36 (haploids), CDC
39 (haploids)
The
START phase occurs during G1 and
after this phase, the cell is committed to
DNA replication and cell division.
Before passing START, cells must obtain a
critical mass.
CDC28 is a critical START p34 protein
kinase whose mutants can block cells at
G1/S or G2 phases.
While
START commits the cell to its life
cycle, mitosis can only take place after
the S phase thus requiring a second
checkpoint in the life cycle, namely the
G2/M phase.
The MIH1 gene speeds up the entry of
cells into mitosis. The role of other genes
in regulating G2/M in S.cerevisiae is
unclear.
Initiation
of M phase depends upon
successful completion of DNA replication
in S phase. The RAD9 gene performs this
function in S.cerevisiae. If DNA
replication is delayed, cells undergo
mitosis with lethal effects.
The p34 kinase increases in activity on
the onset of mitosis and its activity can be
regulated by tyrosine dephosphorylation
at the G2/M stage.
START-specific genes, like CDC 28, act
after DNA replication in G1/S meiosis,
whereas they have an indirect affect on
DNA replication in meiosis.
However, CDC28 is required for the
second G2/M meiosis, where the M and S
phases are uncoupled from each other.
Under
nutritional starvation, yeast cells
stop growing and exit the life cycle (G0
phase).
Unlike in mammalian cells, growth factors
may not play a role in growth control in
yeast cells and such cells in the
stationary (G0 phase) are metabolically
dormant.
Chemical
mutagen
grow colonies
replica plate
identify isolates
Complementation to
identify recessive
lethal mutants
Clone WT gene and
sequence it
Linkage analysis
using Meiotic
analysis.
The
usual methods:
• Random mutagenesis – rapid but matching
phenotypes is slower
• Genetic footprinting – simultaneous testing but
mutant strains can not be recovered
The
new method:
• Delete entire ORFs using PCRs and homologous
recombination
• Direct and simultaneous analysis
• Rapid and increased sensitivity
Deletions of ‘essential’ genes:
• Essential for viability and lacking human
homologues => targets for antifungal drugs
• 356 ORFs identified as essential – failed to grow
(YEPD, 30 oC) as haploid deletants.
• Only 56 % of these previously shown to be essential
for viability.
1620
non-essential genes identified.
Additional homozygous and 2 haploid
deletants constructed.
Non-essential
genes:
• Relatively more complicated to analyze than
essential genes; may require complicated growth
conditions to observe the effects for some.
• 558 homozygous deletion mutants pooled and grown
in Rich (R) and Minimal (M) media.
• Aliquots from both pools Amplify tags
Hybridize to complements on array Hybrid. Data,
measure of growth rate.
• Correlation of UPTAG and DOWNTAG growth rates
(<0.6 of WT for the growth-impaired strains)
New
findings:
• Genes whose inactivation affects growth are not
necessarily the ones induced during growth
under the same particular conditions.
Caveats:
• Neomycin phosphatase (product of KanMX4)
may affect fitness
• Composition of pool, culture conditions
• Complementation (?)
Native GAL4 protein (881aa) contains 2
distinct domains: DNA binding and
Activation Domains
Fuse DB (1-147) with protein X
Fuse portion of AD (768-881) with protein
Y
If X and Y interact with each other in vivo,
DB and AD will be brought together
sufficient enough to activate the AD.
This recruits the transcription machinery
LacZ product is formed.
Caveats:
• Interactions need to occur within yeast
nucleus
• GAL4 Activation region is accessible to
transcription machinery
• BD-X hybrid is itself not an activator
http://www.biologicalprocedures.com/bpo/arts/1/16/m16f1lg.gif