The exo-proteome and exo-metabolome of Nostoc punctiforme (Cyanobacteria) in the presence and absence of nitrate

The ability of nitrogen-fixing filamentous Cyanobacteria to adapt to multiple environments comes in part from assessing and responding to external stimuli, an event that is initiated in the extracellular milieu. While it is known that these organisms produce numerous extracellular substances, little work has been done to characterize both the metabolites and proteins present under standard laboratory growth conditions. We have assessed the extracellular milieu of Nostoc punctiforme when grown in liquid culture in the presence and absence of a nitrogen source (nitrate). The extracellular proteins identified were enriched in integrin beta-propellor domains and calcium-binding sites with sequences unique to N. punctiforme, supporting a role for extracellular proteins in modulating species-specific recognition and behavior processes. Extracellular proteases are present and active under both conditions, with the cells grown with nitrate having a higher activity when normalized to chlorophyll levels. The released metabolites are enriched in peptidoglycan-derived tetrasaccharides, with higher levels in nitrate-free media.