| The marine cyanobacterium Synechococcus is the second most abundant phytoplanktonic organism in the world's oceans. The ubiquity of this genus is largely due to its use of a diverse set of photosynthetic light-harvesting pigments called phycobiliproteins, which allow it to efficiently exploit a wide range of light colors. I have uncovered a pivotal molecular mechanism underpinning a widespread response among marine Synechococcus cells known as "type IV chromatic acclimation" (CA4). During this process, the pigmentation of the two main phycobiliproteins of this organism, phycoerythrins I and II, is reversibly modified to match changes in the ambient light color so as to maximize photon capture for photosynthesis. CA4 involves the replacement of three molecules of the green light-absorbing chromophore with an equivalent number of the blue light-absorbing chromophore, when cells are shifted from green to blue light and the reverse after a shift from blue to green light. I have identified a set of genes that are up-regulated in the Synechococcus cells when the light condition is changed from blue to green and vice versa. Among these, mpeZ RNA is more abundant in blue light, suggesting that its proper regulation is critical for CA4. I have biochemically characterized MpeZ, an enzyme critical for attachment of green light-absorbing chromophore to phycoerythrin II and its isomerization to blue light-absorbing chromophore. In addition, mpeZ mutants fail to normally acclimate in blue light. The genomic region containing mpeZ (or its homologs), along with two transcription factor-like genes, is highly conserved in all the marine Synechococcus strains that elicit the blue-green light acclimation response. My findings provide insights into the molecular mechanisms controlling an ecologically important process and identify a unique class of phycoerythrin lyase/isomerases, which will further expand the use of phycoerythrin in the fields of biotechnology and cell biology. |
