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Dr M.J. Pocklington poc at leicester.ac.uk
Fri Jul 8 13:31:13 EST 1994

Hi buddies and schizos,

The yeast strains I work with (S288C derivatives) germinate "perfectly" 
happily in the absence of added glucose (ie on glycerol YP, galactose YP or 
even YP only plates).  This does not really surprise me, since it would be a
silly yeast beast that missed out on the opportunity to grow on a medium that
it could normally grow on.  What does surprise me is the apparent obsession,
without good reason, with a glucose receptor.

I suspect that the early work identifying glucose as necessary for 
germination, really only showed that a metabolisable carbon source
was necessary for germination. Of course I may be wrong: glucose is a pretty
central point in metabolism, and I suppose that by monitoring (internal, of
course) glucose levels, the cellular growth system could then mount an 
appropriate response. But surely it would make a whole lot more sense if the
germination response was signalled by cyclic nucleotide or NTP or NTP/NDP 
levels, or even a H+ or pH gradient.  Then there would be no need for a 
multiplicity of "receptors"; and such a nutritional status indicator would 
be closer to what really matters in the cell (energy availability in general);
and it is evolutionarily more sensible.  And after all, it seems like thats 
how the starvation response is initiated in the first place.  Perhaps thats
where the hetrotrimeric G-proteins and ras come in (sensing GTP/GDP levels,
consequent upon ATP levels, ultimately).

Evolution in a world of sugar may have selected for yeast that do indeed have 
a glucose receptor.  Fine -lets find it.  But by the above reasoning, not only
is such a receptor unneccessary, but if there was one, it would likely be an
evolutionary speciality -a side issue.

Sensing a return to a happy nutritional status isn't the end of the story as
far as germination is concerned.  All that heat, chemical, dryness, UV, etc.
-resistant packaging has to be undone before the cell cycle can get going 
again, and it shouldn't surprise me if there was an extensive machinery for 
(un)doing it. I bet a mutational approach would discover the genes for 
this/these function(s) rather than those involved in sensing nutritional 

Michael Pocklington


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