GUN4 appeared early in cyanobacterial evolution

Photosynthesis relies on chlorophylls, which are synthesized via a common
tetrapyrrole trunk pathway also leading to heme, vitamin B12, and other
pigmented cofactors. The first committed step for chlorophyll biosynthesis
is insertion of magnesium into protoporphyrin IX by magnesium chelatase.
Magnesium chelatase is composed of H-, I-, and D-subunits, with the
tetrapyrrole substrate binding to the H-subunit. This subunit is rapidly
inactivated in the presence of substrate, light, and oxygen, so oxygenic
photosynthetic organisms require mechanisms to protect magnesium chelatase
from similar loss of function. An additional protein, GUN4, binds to the
H-subunit and to tetrapyrroles. GUN4 has been proposed to serve this
protective role via its ability to bind linear tetrapyrroles (bilins). In
the current work, we probe the origins of bilin binding by GUN4 via
comparative phylogenetic analysis and biochemical validation of a
conserved bilin-binding motif. Based on our results, we propose that
bilin-binding GUN4 proteins arose early in cyanobacterial evolution and
that this early acquisition represents an ancient adaptation for
maintaining chlorophyll biosynthesis in the presence of light and oxygen.