The marine cyanobacterium Prochlorococcus is the smallest and most abundant photosynthetic organism in the ocean, contributing substantially to global primary production. In Prochlorococcus, CO₂ is fixed through the Calvin cycle to generate glucose during the day, which is subsequently consumed via the pentose phosphate pathway for nucleotide synthesis at night. This day-night metabolic transition is tightly regulated by the small protein CP12, which inhibits Calvin cycle activity by forming an inhibitory complex with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK). 

Interestingly, cyanophages infecting Prochlorococcus have also been found to encode and express CP12, despite lacking other Calvin cycle genes and the conventional association of CP12 with viral processes. To investigate the function of phage-encoded CP12, we conducted a series of protein purification and interaction assays. The results demonstrated that phage CP12 is capable of functionally replacing host CP12 and can bind to host GAPDH, suggesting its potential involvement in modulating host carbon metabolism during infection.

These findings shed light on a previously unrecognized strategy employed by cyanophages to influence host metabolic pathways and highlight the ecological relevance of auxiliary metabolic genes in viral manipulation of microbial primary production.