column chromatography to constant specific 

 activity; the total counts per minute attributable 

 to this purified glucose are shown in the lower 

 left hand figure. The two sets of data, when 

 combined, yielded the specific activities of the 

 glucose pools in RS cells of different ages - in- 

 cluding reversals - shown in the upper figure. 

 By way of summary, morphological reversal 

 before the point of no return does, indeed, bring 

 about an increase in the specific activity of the 

 glucose in the cell's free pool; after the point 

 of no return, this does not occur. Once again, 

 although these results could be interpreted in 

 several ways, they are consistent with the theory 

 we have been discussing. 



Obviously, much more work is needed, and 

 we are in the process of doing some of it. In 

 conclusion, Blastocladiella emersonii provides 

 a very satisfactory system for studying the 

 relations between biochemical and morphologi- 

 cal differentiation (2). I am sure that Dr. Lovett, 

 who now follows me on this program, will illus- 

 trate in yet another way that this is so. 



SCHRAER: What is the ecology of this 

 organism? 



C ANTING: It is found in fairly slow-moving 

 bodies of water, sometimes streams but more 

 often ponds and puddles, and in soil. The genus 

 is ubiquitous, but for this particular species I 

 cannot say definitely. But what do you mean by 

 ecology, in particular? 



SCHRAER: I was referring to the oxygen 

 content of the water in which they are found. 



CANTINO: It likes to grow in fairly well 

 aerated bodies of water, but it is unhappy, 

 apparently, in bodies of water heavily laden with 

 organic matter. It can be trapped on insect 

 exoskeleta, fruits of the Rosaceae, etc. 



ZIMMERMAN: Does the CO2 content in 

 these waters vary sufficiently to give you the 

 either/or type of growth? 



CANTINO: I think so, but not directly be- 

 cause of the CO2 content of the water. Rather, 

 it is the fact that when this organism grows in 

 nature, it is often surrounded by microflora 

 and fauna which tend to localize around it. I feel 

 certain, although I've never stuck microelec- 

 trodes therein, that the CO2 concentration with- 

 in such localized "pockets" must be higher than 

 on the outside, and that this may be involved in 

 the induction of RS differentiation in nature. 



EPEL: Does isocitrate eliminate CO2? 



CANTINO: Isocitrate did not do so in ex- 

 periments done years ago; we'd like to assume 

 that this was because it does not get in very 

 easily. Ketoglutarate does get in, and does the 



trick under certain conditions of nutrition. 

 Presumably, it functions as a substrate for the 

 backward reaction. 



EPEL: Have you been able to use malic 

 acid to affect CO 2 consumption? 



CANTINO: Do you mean the Ochoa "malic" 

 enzyme? We think it may be operative at some 

 stages, but I would not care to say "yes" or "no" 

 because these assays were done about ten years 

 ago when we were not using synchronized cul- 

 tures. Therefore, I'mnot sure of the significance 

 of these old assays. 



KAHN: How do you visualize the control of 

 enzyme levels or activity? 



CANTINO: I have no satisfactory basis for 

 speculation on this point. 



CHALKLEY: Is there DNA synthesis at the 

 point of no return? 



CANTINO: Net DNA synthesis ceases at 

 the point of no return. If we plot the DNA/cell 

 (using older methods of extraction of some 6-7 

 years ago) against developmental age, the curve 

 rises and then levels off almost precisely at 

 the point of no return. There is no additional net 

 synthesis of DNA after this point, although net 

 synthesis of RNA continues. However, during the 

 period after the point of no return, the composi- 

 tion (base ratios and physical properties) of the 

 RNA begins to change, (see subsequent reply by 

 CANTINO to a question by GROSS). 



J. WRIGHT: I gather. Dr. Lovett, that 

 you're going to discuss this in the next paper? 



LOVETT: Yes, but I won't discuss that part 

 of the life cycle. 



KAHN: Is the DNA synthesis synchronous? 



CANTINO: If you mean synchronous in 

 terms of nuclear division, we don't know. For 

 RNA, we have more data, but I would rather not 

 go out on a limb even here. 



GROSS: Are there mutants of this organism 

 that are incapable of making the switch? 



CANTINO: Yes, during the past 18 years 

 we've isolated four spontaneous mutants during 

 essentially day-to-day observations of cultures 

 growing on plates. So, they occur with a low 

 frequency. The mutants are incapable of re- 

 sponding to the bicarbonate trigger mechanism 

 or, as far as we know, to any other "inducer"; 

 they will not form BS. When some were analyzed 

 for their enzyme contents, they were found to 

 lack ketoglutaric dehydrogenase (and aconitase). 



GROSS: Do they lack it entirely? They don't 

 have the characteristic response? 



CANTINO: No, they won't respond; they 

 won't produce RS in response to bicarbonate. 

 (Note added in proof: The mutants on which 



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