Responses of unicellular cyanobacteria to changes in their carbon regime
The rate of inorganic carbon (Ci) uptake and RuBisCO activity was examined in batch culture, in a range of unicellular cyanobacteria grown under different carbon regimes. There was no significant increase in RuBisCO activity seen in CO2-limited cultures. All of the species capable of growth in freshwater were found to possess, to varying degrees, an active, saturable, mechanism for the transport of Ci. Three oceanic, phycoerythrin containing unicellular cyanobacteria did not possess a Ci uptake mechanism following growth under high and low CO This may be related to the nature of the environment which these organisms inhabit. Addition of sodium bicarbonate and glucose to low CO2-grown Synechocystis PCC6803 resulted in a rapid decay in Ci uptake, with a t0.5 for the decay of 1.15 h. Addition of the non-metabolizable glucose analogues 3-0-methyl-D-glucase (OMG) and 2-deoxy-D-glucose to low CO2-grown Synechocystis PCC6803 resulted in different effects on the cells ability to transport Ci. Addition of OMG resulted in no significant decline in the Ci uptake mechanism, whereas addition of DOG led to a similar decay in Ci uptake to that seen following addition of glucose. From these results it is suggested that the different structures of the analogues are responsible for the differences seen, and that build-up of a pool of glucose-6-phosphate is a signal for the inactivation of the Ci uptake mechanism in Synechocystis PCC6803. Examination of the Ci uptake mechanism and RuBisCO activity in DIC and light-limited chemostats of Synechococcus PCC7942 resulted in the finding that cultures which were DIC-limited for growth were not DIC-limited with respeqt to the Ci uptake mechanism. RuBisCO activity increased in three distinct stages as the external DIC concentration fell. The 42 kD cytoplasmic membrane polypeptide was present in organisms isolated from DIC-limited chemostats with no appreciable Ci uptake mechanism. The in vivo and in vitro phosphorylation pattern of Synechocystis PCC6803 was also examined in response to growth under different carbon regimes. Low CO2-grown cultures contained few phosphopolypeptides. Addition of sodium bicarbonate and glucose to low CO2-grown cells led to the appearance of a number of phosphopolypeptides over a similar time course seen to the decay of the Ci uptake mechanism. The glucose analogues DOG and OMG had no effect an the phosphorylation pattern in low CO2-grown cells. In vitro kinase experiments an cell-free extracts resulted in a number of polypeptides becoming phosphorylated, some corresponding on SDS-PAGE to those seen in vivo. The in vitro phosphorylation pattern could be requiated by altering the redox potential and addition of RUBP. At the molecular level, the construction and use of the lacZ promoter probe pLACPB2 led to the identification of a number of presumptive CO2-regulated promoters.