SUMMARYThe lengths of organisms of Escherichia coli B/r grown in continuous culture in a glucose salts medium were measured over a wide range of population densities at 22' and 37'. In some cases the cultures were exposed to continuous gamma radiation at dose rates of 600 r./hr. at 22" and of 1000 r./hr. a t 37". The average length of the forms and the distribution with respect to length depended upon the temperature, population density and radiation dose rate. The growth rate was almost independent of population density over the range in which the average length of organism showed such a marked dependence. No significant differences in growth rate were observed as between unirradiated and irradiated cultures. Changes in average length of organisms are regarded as phenotypic responses to changes in growth conditions, some conditions favouring division and others inhibiting division amongst short organisms.

A number of pteridine derivatives stimulate photosynthetic phosphorylation in spinach chloroplasts. In general, tetrahydro or dihydro compounds are highly active, as is one aromatic, naturally occurring compound, biopterin. The physiological characteristics of this photosynthetic phosphorylation are the same as those described for phosphodoxin. A pteridine-containing fraction from spinach, with the same R(F) value as phosphodoxin, is also active.

JANSZ, E. R., and F. I. MACLEAN. 1973. The effect of cold shock on the blue-green alga At~acystist~idlrlat~s.Can. J. Microbiol. 19: 381-387. We have investigated the known susceptibility of the blue-green alga Atlacystis tlidilatzs to cold shock. After suspensions of the organism were exposed to temperatures of 0"-3" for 15 min, viability was reduced to about 2% and the photosynthetic rate to about one third. Resistance to this effect increased with age of the ci~lti~re from which the cells were harvested. Polarographic measurements showed that the decline in photosynthesis was cornplete within a few minutes after removal from the cold, and autoradiography showed that the decline was distributed anlong all the cells in the suspension. The short-term 14C02 fixation products in cold-shocked and control cells differed only in amount and the 14C02 fixed/Oz evolved during photosynthesis was the same in control and cold-shocked cells. Cold shock also caused the loss of some of the diffus~ble material fro111 the cell; the loss was rapid but did not continue beyond about one quarter of the diffusible C and P ; intracellular glutamate was lost to the extent of 50-80%. It is suggested that cold shock affects the limiting membrane of the cell and that this causes cell death but it is uncertain whether it is also responsible for the decline in photosynthesis.JANSZ, E. R., et F. I. MACLEAN. 1973. The effect of cold shock on the blue-green alga AnacysfL tzidtlntrs.Can. J. Microbiol. 19: 381-387. On Ctudie la susceptibilitt connue de l'algue bleu-vert Anacysfis nidrtlatrs au choc au froid. Quand des suspensions de cet organisme sont expostes aux temperatures de 0"-3" C pendant 15 min, la viabilitt des cellules est reduite a environ 27, et la vitesse de la photosynthkse a environ 30%. La resistance a cette action augmente avec 1'8ge de la culture. Quelques minutes aprts I'enltvement des cellules au froid, les mesurages polarographiqi~es lnontrent que la diminution de la photosynthtse est complkte, et I'autoradiographie montre que la diminution se manifeste parmi toutes les cellules de la suspension. Les produits de la fixation I4Co2, pendant une courte periode, chez les cellules ayant subit un choc au froid et chez les cellules tenloins ne different qu'au point de vue quantitatif. Le rapport du l4CO2 fixt/Oz dtgagt pendant la photosynthtse est le n~&me dans les cellules temoins que dans celles ayant subit un choc au froid. Le choc au froid est aussi responsable de la perte d'une partie des substances diffusibles des cellules. cette perte est rapide nlais elle ne se prolonge pas au-dela de 25% du C et P diffusibles. I1 y a perte de 5&80% du glutamate intracellulaire. On suggtre que le choc au froid affecte la membrane cellulaire et que cet effet cause la mort de la cellule, mais il n'est pas certain que c'est aussi la cause de la diminution de la photosynthtse.[Traduit par le journal]

Cell-free extracts of A. nidulans show high rates of cyclic and noncyclic photophosphorylation and P/2e ratios of about 0.8. These extracts also reduced ferricyanide at rates as high as 700 μmol/mg chlorophyll per hour. Rates of dichlorophenol indophenol (DCPIP) reduction and reduced cytochrome c oxidation similar to those previously reported in the literature have also been observed. The extracts, however, showed no NADP-reducing capacity. Subjecting cell-free extracts to cold shock caused no decline in electron flow but a marked decline in photophosphorylation. The sensitivity of photosynthesis of this organism to cold shock (1, 2) might be explained, at least to some extent, by this decline in photophosphorylation.

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