Figure 5.

Chlorophyll fluorescence in Anabaena cultures 2 days after the addition of 2 μM Cu²⁺ to induce expression of the hetR gene. In each case, the culture contained wild-type Anabaena carrying the plasmid pPetHetR, and the fluorescence was viewed with a wide bandpass filter. (A) The culture contained 1 mM (NH₄)₂S0₄. (B) The culture contained 17 mM nitrate. (C) The culture contained no fixed nitrogen. The heterocysts can be identified by the low fluorescence attributable to the loss of chlorophyll from photosystem II and the degradation of the phycobiliproteins.