Lecture 4 - Feb 2.docx

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19 Apr 2012
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Lecture 4 Feb 2
CSB429
No Recording, ask Emily or Ying
Polar granules = large conglomerates of proteins and particles (mRNP particle)
- We don’t understand all features of these particles in the germ plasm, they are composed of
many proteins and RNAs; but it is clear they are important in making the germ line
Some molecules important in making germ lines: Oksar, Vasa, Tudor, etc. And they recruit other
proteins and RNAs that include Germcell-less, Nanos, Pgc, etc. And these molecules are important in
either making the GLCs or maintaining them
Interesting enough, there is some mitochondrial RNA (mtRNA) that is important for germ cell formation
In picture on slide, the granules are in close association to mitochondria and apparently some of the
mitochondrial proteins leave mitochondria and enter cellular cytoplasm to form mtRNAs (normally
mtRNA is within the mitochondria and is important for mitochondrial proteins) but here they are part of
the germ plasm and seem to be important in the translation of some cellular proteins
How do these mitochondrial ribosomes and RNA get out of the mitochondria? No one really knows.
We already talked about nanos: it is important because if we lose nanos, we lose germ cells, and these
cells can actually take on somatic cell roles
- So nanos is obviously very important in these cells becoming germ line cells
- Even if all other factors are present, you need nanos to have germ line cells
Nanos is a generally translational repressor and it is not completely understood what the direct targets
of nanos are (but we come back to this in a few minutes)
Pgc these molecules are important to inactive the RNAP2 so it interferes with transcription
Analysis of these molecules has shown that some of these molecules are involved in repression of
transcription (which includes pgc, which directly inavtivates the RNAP2; or nanos and germcell-less)
- So these three (and maybem ore) lead to a transcription silencing in the germ line
- During early stages of germ line , they are completely transcriptionally silent
- Transcription silencing is important in preventing the somatic cell identification / fates
There are other molecules: Vasa, Tudor and mtRNAs that regulat translation and they are important in
producing germ line producing factors
Idea that has developed is that to make a germ line, you need, on one hand, vectors that promote germ
line development (little known) and we need to suppress somatic cell fate
Timeline:
Hours
1.5 stage 4 (formed pole cells)
2 stage 5
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3- - begin gastrulation
3.5 hours toward end of gastrualtion
Transcriptional activity
In all cells of embryo, no transcriptional activity in first 2 hours the egg is full of nutrients, RNA,
(maternal factors) from mom that keep embryo going
Soma slight increase in transcriptional activity from 1.5 to 2 hours, but after 2 hours = lots of
transcriptional activity (zygotic transcription)
PGCs 3.5 hours = transcription begins
- If we take nanos or pgc away; germ line is disrupted so the silencing until 3.5 hours is anecessar
So pictures:
2 Hour embryo = will have pole cells on one end and cells encompassing the embryo; these cells are
undergoing zytgotic transcription while the pole cells remain silent
6:38
In germline, even when RNAP2 is able to allow transcriptional elongation, there is no H3K4 methylation
so the DNA remains wrapped up in histones (so closed chromatin you need open chromatin regions
to transcribe)
- Nanos is responsible for this H3K4 methylation
- Same for GcL
2 important mechanisms that contribute to global silencing and is important for germ line development
The soma is developing, making proteins, very active and our germ lines are sitting there and doing
nothing? Is that true?
Well actually, if we look at transcription level it is true, but the translational level it is very different
So what happens to RNAs in early embryo?
So we actually have very active process of posterior localization of maternal mRNAs
- Very complex process and a lot of work is done to study the transport
- Will develop into the germplasm and where germ cells will develop
In addition, we have ubiquitous mRNAs that are evenly spread throughout the embryo
In this early embryo, no transcription; theses are all maternal mRNAs
At the 2 hour stage (when transcription starts in soma) a number of these maternal mRNAs are
degraded in the soma but in the pole cells (germ lines), these maternal mRNAs are stabilized and
preserved
Sp we have the blastoderm there is degradation of the mmaternal mRNAs but in the germline, there is
protection of these mRNAs
So these germ lines are pumped full of maternal mRNAs and proteins
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