From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Wed Jul 24 2002 - 00:49:56 MDT
On Tue, 23 Jul 2002, Harvey Newstrom, commenting on my comments, wrote:
> Actually, it doesn't work that way. DNA is more like a blueprint with
> instructions. Having duplicate copies of the blueprint doesn't turn a
> cottage design into a mansion. If you have the recessive genes for blue
> eyes, duplicating the genes won't make your eyes more blue. You still
> have blue eyes.
I don't disagree with the analogy with respect to many structural
features. Doubling the homeobox genes for a "thorax" region does
not generally give you two "thorax" regions. However you can add
"antenna" genes under the control of different regulatory factors
and get additional antenna to appear in alternate regions of the body
(at least in Drosophila).
> To store more beta-carotene, you would need to increase
> production, expand storage structures, or eliminate regulatory
> processes. Just duplicating existing genes won't do it.
Beta-carotene is generally considered to be a lipid-soluble
vitamin. So long as you don't saturate the lipids with BC
at so high a level that you change their physical properties
you do not have to worry about storage structures. The genes
were added under the control of the glutelin promoter or the
constitutive 35S promoter. A detailed analysis would be
required to determine if the protein abundance produced
by one of those promoters was rate limiting. If so, then the
multiplying the abundance of the rate limiting protein would
increase the production of BC. If not, then multiplying
the number of copies of the entire pathway (as I previously
suggested) would be the correct approach. Only in the event
of source substrate limitation (i.e. limits of the # of molecules
on which the pathway acted), would this not work. In such a case
one would need to add more enzymes or entire pathways to produce BC
precursors.
BC is, to my knowledge, a CHON dependent molecule, so there should
be no external limits for its production (unlike say hemoglobin
which requires Fe), only internal limits on access to substrate
to convert to BC *or* pathways to convert abundant molecules
(e.g. glucose or fats) into BC.
Going back to Harvey's analogy, if the blueprint has multiple
copies of a bedroom design, you *do* get more bedrooms and
you do have the capacity to sleep more people. The guests
(BC precursors) don't particularly care if all the bedrooms look
the same. They care whether or not they have a bed to sleep in.
Robert
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