rubrum and a similar correlation was shown for a number of

phytoplankton species (Glover and Morris, 1979). In contrast,

Beudeker et al., (

198

by whole cells of T. neapolitanus and Karagouni and Slater (1979) found that in neither light-nor carbon dioxide-limited chemostat cultures of A. nidulans did the changing pattern of carbon dioxide fixation by intact organisms correlate with the observed

RuBPCase activities. Karagouni and Slater (1979) also reported

that the RuBPCase activity in vitro accounted for only 10?S of the in vivo rate of carbon dioxide assimilation required for the growth of A. nidulans at high dilution rates so reflecting the problems of correlating enzyme activity with the behaviour of intact organisms.

Growth rate may affect RuBPCase activity in certain organisms. In thiosulphate-limited cultures of T. neapolitanus a slight

decrease in activity was measured with increasing growth rate although in T. pdophila RuBPCase activity was almost independent of growth rate over a wide range (Kuer.en and Veldkamp, 1973)« Slater (1975) reported a four-fold increase in RuBPCase activity on a per cell basis for light-limited chemostat cultures of A. nidulans over the growth rate range 0.02 - 0.10 h-1 although in terms of total protein the activity remained constant at

these growth rates. In contrast Karagouni (1979) and Karagouni

and Slater (1979) reported that under the same conditions of light-limitation over the growth rate range 0.02 -

0.19

RuBPCase activity remained constant both in terms of total protein and unit cell number. These workers also reported a 15-fold increase of RuEPCase activity on a protein and unit cell basis in carbon dioxide-limited cultures of A. nidulans with decreasing

growth rate. It was concluded that the alterations in the

RuBPCase activity was probably in response to changes in the external carbon dioxide concentration rather than different growth rates since under light-limited conditions with carbon dioxide in excess at all dilution rates examined there was no change in the specific activity of the RuBPease. The results also suggested that this enzyme was under transcriptional control in this

organism in contrast to the suggestion of Carr (1973a) that cyanobacteria cannot in general control their enzyme activities at the transcriptional level and as a consequence are obligate autotrophs.

Temperature was also found to affect the RuBPCase activity

of A. nidulans (Karagouni, 1979)* Low temperature (25°C rather

than 40°C) was found to cause an increase in the specific activity of the RuBPCase under both light-and carbon dioxide-limitations. In addition the specific activity under light-limitation was

found to increase with increasing growth rate rather than remaining constant as at k O ° C . The rate of carbon dioxide assimilation

was found to be depressed at low growth temperatures so it was concluded that the increased RuBPCase levels at temperatures lower than optimal could presumably scavenge the available carbon dioxide in a similar manner to that shown at low carbon dioxide

concentrations when a high specific activity of RuBPCase was also found.

Light intensity may affect RuBPCase activity although reports are contradictory. Lascelles (i

960

Merrett, 1972). The age of a culture m a y also affect the RuBPCase

activity which was shown to increase in T. intermedius (Purohit, McFadden and Shaykh, 1976b) and Paracoccus denitrificans (Shively, Saluja and McFadden, 1978) and remain stable in Thiobacillus denitrificans (McFadden and Denend, 1972) during the exponential growth phase and then decrease slightly during stationary phase. Similar results were obtained for the cyanobacteria Anabaena CA and A. quadruulicatum with the RuBPCase steadily increasing during exponential growth of these organisms (Tabita, Caruso and Whitman, 1978).

Generally when bacteria are grown autotrophically they contain RuBPCase in large amounts with a high specific activity but when grown heterotrophically this activity is usually drastically reduced as shown for T. novellus (Aleem and Huang, 1965), T . intermedius (Purohit et al.. 1976b), R. spheroides (Lascelles, i

960

heterotrophic mode of growth a relatively slow decline of RuEPCase was shown, there was no enzyme detectable after

2 b hours. However, under some conditions of heterotrophic

culture, high rates of RuBPCase were sustained, for example, in Hydro<renomonas facilis and Hydroscenomonas eutrooha when grown on fiuctose when the aeration was low, FeCl^ added and the cells harvested prior to or at the mid-exponential phase of growth (Kuehn and McFadden, 1968) and in R. rubrum when grown on butyrate in the light (Tabita and McFadden, 197*0« On the other hand, under normal heterotrophic growth conditions the RuBPCase activity in cell-free extracts of the obligate chemolithotrophs, T. pelonhila, Thiobacillus thioparus and T. neapolitanus, was independent of the presence of exogenois

organic compounds,such as acetate and succinate (Kuenen and

Veldkamp, 197?). No change in the enzyme could be demonstrated

either in thiosulphate-or carbon dioxide-limited cultures. These organisms were shown to lack an operative tricarboxylic acid cycle and glyoxylic acid cycle as do cyanobacteria (section 1.1.3.) (Smith, London and Stanier,

1967

compounds. Pearce and Carr (

1967

which indicated that RuBPCase activity in A. nidulans and A. vadabilis was only slightly altered after growth in the presence of sodium acetate and Harrison and Carr (unpublished observations quoted in Carr, 197?b) found that neither glucose

1

nor pyruvate repressed RuBPCase activity in A. vgjdabilis. Harrison and Carr (unpublished observations quoted in Carr, 1973a) also found no alteration of RuBPCase activity in

cyanobacteria after the inclusion of sugars in the phototrophic growth medium and reported that RuBPCase activity in dark heterotrophic cultures of Chlorogloea fritschii was still the same as in phototrophic cultures after 8 weeks of growth. However, after a period of one year's maintenance in the dark

the activity had dropped to approximately 7 % of the phototrophic

culture. This observed decline in enzyme activity could possibly be accounted for due to the fact that the growth rate of the organism under dark heterotrophic conditions was at least 10-fold

less than in the light. Joset-Espardellier, Astier, Evans and

Carr (1978) working on Aohanocansa 671il and C. fritschii also showed little variation in enzyme level under photoautotrophic,

photoheterotrophic and heterotrophic growth conditions. In both

organisms the RuBPCase activity decreased slightly under photo­ heterotrophic growth conditions but actually increased under heterotrophic growth conditions in C. fritschii although remaining constant in Abhanocapsa 671^ as under photoautotrophic growth

conditions. In contrast to these results, Carr, Hood and Pearce

(I

969

acetate in the growth medium of A. variabilis caused a reduction in the RuBPCase activity compared with the autotrophically grown control organisms.

1.1.2.2. (3-carboxylations

B-carboxylations catalyse the conversion of C0 to C,L