Cyanobacterial psbA families in Anabaena and Synechocystis encode trace, constitutive and UVB-induced D1 isoforms.

Cyanobacteria cope with UVB induced photoinhibition of Photosystem II by regulating multiple psbA genes to boost the expression of D1 protein (in Synechocystis sp. PCC6803), or to exchange the constitutive D1:1 protein to an alternate D1:2 isoform (in Synechococcus sp. PCC7942).