The chlL gene encodes one subunit of the light-independent protochlorophyllide reductase. A chlL-lacking mutant of the cyanobacterium Plectonema boryanum is unable to synthesize chlorophyll (Chl) in the dark, causing Chl synthesis to become light-dependent as in angiosperms. When the mutant cells were cultivated heterotrophically in the dark, Chl synthesis was arrested and the Chl content decreased exponentially in reverse profile to cell propagation, indicating that most of the pre-existing Chl was recruited for daughter cells. During this 'etiolating' process the Chl content became less than 0.5% of the original level. In parallel to this there was a decrease in the activity of Photosystem I (PSI), the amount of its core Chl-binding subunits, PsaA/PsaB, and a peripheral subunit, PsaC. Levels of transcripts for these subunits were not significantly changed upon the arrest of Chl synthesis. In contrast, Photosystem II (PSII) was maintained to a significant extent in terms of activity and protein levels of D1 and CP47 until a late stage of the etiolation, implying that PSII is newly synthesized though Chl synthesis was arrested. Low-temperature (77 K) fluorescence spectral analysis supported a selective decrease in Chl associated with PSI. Taken together, it is suggested that the pre-existing Chl molecules in periphery of PSI could be released and re-distributed for PSII biosynthesis in the etiolating cyanobacterial cells.