Abstract:
By using glucosamine resistant mutants of
Saccharomyces ceriv~sa~ an attempt was made to discover
the mechanisms which cause glucose repression and/or the
Crabtree effect. The strains used are 4B2, GR6, lOP3r,
GR8l and GRI08. 4B2 is a wild type yeast while the others
are its mutants. To characterize the biochemical
reactions which made these mutants resistant to glucosamine
poisoning the following experiments were done~ 1. growth
and respiration; 2. transport of sugars; 3. effect of
inorganic phosphate (Pi): 4. Hexokinase; 5. In yivo
phosphorylation.
From the above experiments the following conclusions
may be drawn:
(i) GR6 and lOP3r have normal respiratory and
fermentative pathways. These mutants are resistant to
glucosamine poisoning due to a slow rate of sugar transport
which is due to change in the cell membrane.
(ii) GR8l has a normal respiratory pathway.
The slow growth on fermentable carbon sourCEE indicates
that in GR8l the lesion is in or associated with the
glycolytic pathway. The lower rate of sugar transport may
be due to a change in energy metabolism. The invivo
phosphorylation rate indicates that in GR81 facilitated
diffusion is the dominant transport mechanism.
(iii) GR108 msa normal glycolytic pathway but
the respiratory pathway is abnormal. The slow rate of
sugar transport is due to a change in energy metabolism.
The lower percentage of in vivo phosphorylation is probably
due to a lowered availability of ATP because of the
mitochondrial lesion.
In all mutants resistance to glucosamine poisoning
is due to a lower rate of utilization of ATP. which is
caused by various mechanisms (see above), making less ADP
available for phosphorylation via ATP synthase which
utilizes inorganic phosphate. Because of the lower utilization
of Pi, the concentration of intra-mitochondrial Pi
does not go down thus protecting mutants from glucosamine
poisoning.
