This study investigated glutamate synthase (GOGAT) activity in heterocysts versus vegetative cells of the cyanobacterium Anabaena variabilis. Using radioactively labeled substrates, researchers measured GOGAT activity in cell-free extracts from isolated heterocysts and whole filaments. When using labeled glutamine as substrate, heterocysts produced only about 17% of the glutamate detected in whole filaments, even without adding cofactors required for GOGAT activity. Critically, when using labeled 2-oxoglutarate (an alternative GOGAT substrate), heterocyst extracts showed negligible glutamate production in the absence of reduced ferredoxin, accounting for at most 3% of whole filament activity. The authors concluded that virtually all glutamate production from glutamine in heterocyst extracts occurs through GOGAT-independent pathways, likely via glutaminase activity. These findings resolve conflicting earlier reports and suggest that heterocysts possess minimal to undetectable GOGAT enzyme activity, despite their role in nitrogen fixation and ammonia assimilation in these filamentous cyanobacteria.

Key findings

• Heterocyst extracts showed only 3% or less of the GOGAT activity present in whole filament extracts when measured using 2-oxoglutarate as substrate
• Glutamate production from glutamine in heterocysts occurs primarily through GOGAT-independent mechanisms, likely glutaminase activity
• GOGAT activity is negligible or undetectable in heterocysts despite their high glutamine synthetase activity and role in nitrogen fixation
• Results clarify conflicting previous reports about GOGAT presence in heterocysts of different Anabaena species