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An abstract of the dissertation of Abbas Sadeghi for the Doctor of Philosophy in Environmental Sciences and Resources: Biology presented on May 7, 1998. Title: Gene expression in two cyanobacteria, freshwater Synechococcus sp. PCC 7942 and oceanic Synechococcus sp. WH 7803, in response to ammonium, nitrate or iron. The transcriptional response of freshwater Synechococcus sp. PCC 7942 and oceanic Synechococcus sp. WH 7803 to ammonium, nitrate or iron was studied in single or multiple factor limited cultures. Both strains showed maximum production of NiR mANA when grown in nitrate-containing media. When grown in ammonium-containing medium, they did not show any signal for NiR mRNA synthesis. The influence on the transcription of NiR mRNA by iron as sole limiting nutrient was also evaluated. Iron increased the NiR mRNA whether or not the positive effect of nitrate was already present. The hybridization signal of mRNA for the large subunit of ribulose bisphosphate carboxylase/oxygenase enzyme (rbcl), was increased in both strains when iron was added. The relative response of NiA was larger than that of rbcL to iron addition. In the final set of experiments, combinations of iron, ammonium, or nitrate nutrient limitation were studied to understand their interactive effect on NiA and rbcL mANA production. The ammonium-grown cells were allowed to starve for iron and then transferred to +iron+nitrate, +iron-nitrate, or -iron+nitrate cultures. rbcL mANA was increased gradually up to 168 hr. NiA mANA increased initially but, for unknown reasons, decreased after 24 hr to a minimum that was similar for +iron+nitrate, +iron-nitrate, and -iron+nitrate cultures. A comparison of the rbcL or NiA mANA from multiple nutrient limitation with that of single nutrient limitation showed different time dependent patterns of synthesis. The response to iron starvation could be different when cells experience a co-limitation with nitrate. This response is also influenced by species specific differences by evolutionary adaptation to different environments. This difference is confirmed in the NiR and rbcL mANA response in two strains of marine and freshwater Synechococcus for single (iron) or multiple (iron+nitrate) limitation. Physiological studies commonly assume a single factor will constrain cell growth at any moment and when conditions change, the limiting factor will switch. The differential response in the synthesis of NiA and rbcl mANA observed shows that cells exhibit unique responses to combined limitations. GENE EXPRESSION IN TWO CYANOBACTERIA, FRESHWATER SYNECHOCOCCUS SP. PCC 7942 AND OCEANIC SYNECHOCOCCUS SP. WH 7803, IN RESPONSE TO AMMONIUM, NITRATE OR IRON

Gene Expression in Two Cyanobacteria, Freshwater Synechococcus sp. PCC 7942 and Oceanic Synechococcus sp. WH 7803, in response to ammonium, nitrate or iron