776 Editor: E. HEINZ 
it. In fact a rigid control of the quantity associated with the carrier, if it is in equilibrium with S, 
might also rigidly control the concentration of compounds in S. 
The equilibria we are discussing here are complicated by many chemical transformations also 
going on. The evidence that the compounds in S are in fact organized in some special way in this 
cell is provided by experiments with osmotic shock. Every pool we have measured in EF. coli—as 
distinct from the pools in yeast—is osmotically sensitive. If cells are washed with water at room 
temperature the large pool (S) of uracil compounds as well as the amino acid pools are totally 
extracted. However, if the cells are washed with water at 0° the amino acids are still totally ex- 
tracted while the uracil compounds are only partly extracted. This shows quite clearly a difference 
in organization in the cell between the two different kinds of pools. 
LajtuHa: I would like to use the structures of Dr. BRITTEN to illustrate alternative interpretations 



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Fig. 6. Incorporation of [2-!C|-uracil into the RNA of E. coli ML-30 after 10 min pretreatment 
with [?2Cj-uracil at 4- 10 ® M. The upper figure presents the data at early times with expanded 
scales. The light solid and dashed curves represent the best separation of the curve into a linear 
component and a component delayed by the pool (S). Solid circles are experimental values. 
For the open circles the component delayed by S was subtracted. The abscissa scale is proportional 
to increase in cell mass, with sample times indicated. 
of some of his experimental data. Such data, though they may be consistent with the existence of 
heterogenous substrate pools do not necessarily indicate such pools, if the enzymes metabolizing 
the substrate are spatially oriented in a special way. Though I am thinking of amino acid metabo- 
lism, the compounds on the blackboard can be used as an example. If the enzymes synthesizing 
RNA were at the surface or on a structure which is preferentially accessible from the surface, then 
labeled RNA precursors added from outside would be incorporated into RNA at a higher rate 
than endogenous precursors. In spite of this preferential incorporation, however, there could still 
be a homogenous precursor pool in the cell. Whenever in short term experiments the labeled sub- 
strate in the cell has lower specific activity than its metabolic product, a compartmentation of the 
substrate can be deduced. 
BRITTEN: But would you go so far as suggesting that the concentrating process, incorporation 
into RNA, internal synthesis, and the chemistry for conversion from uracil to nucleotide phosphate, 
that all of these can actually occur in the membrane region? Although I did say, that this was 
beyond the stretch of imagination, I think now it could be visualized. 
LajTHA: I agree with Dr. BritTeEN that in his system this is unlikely. We have, however, some 
evidence that for hippurate synthesis, which involves mitochondrial enzymes, glycine from plasma 
is used in preference to endogenous organ glycine. This might be explained equally well by com- 
partmentation, in a sense, of the enzymes as by heterogeneous glycine compartments. 
Baxter: I would like to ask Dr. GuroFF a question relating to his work with tyrosine in brain. 
Much emphasis at this symposium has been placed upon processes involving incorporation of amino 
References p. 777 
