iTRAQ-based quantitative proteomic analysis of Gloeothece sp. PCC 6909: Comparison with its sheathless mutant and adaptations to nitrate deficiency and sulfur limitation

Gloeothece sp. PCC 6909 is a unicellular N(2)-fixing cyanobacterium with a well defined and highly developed sheath surrounding its cells. A sheathless mutant of this strain was previously obtained by chemical mutagenesis and, although lacking the sheath, it releases large amounts of polysaccharides into the culture medium. To provide a global understanding on the metabolic differences between the two phenotypes, the proteomes of the wild type and mutant were analyzed using a cross-species proteomics approach coupled with iTRAQ isobaric tagging technology, since their genome sequences are not yet available. Effects arising from the presence/absence of nitrate and sulfur are presented as two metabolically directed follow-up iTRAQ studies. These nutrients are believed to play a major role in Gloeothece's metabolism, including the production of extracellular polymeric substances - EPS. 454, 124, and 53 proteins were identified and reliably quantified using homology anchoring approaches for iTRAQ previously described. The results obtained strongly suggest that the chemical mutagenesis affected the regulation of a number of key cellular processes, as revealed by the significant fold changes observed for proteins covering a large spectrum of functional groups. Moreover, they provide new insights on the adaptations of Gloeothece cells to nitrate-deficiency and sulfur-limitation.