PII signal transduction protein in Chlamydomonas reinhardtii: localization and expression pattern.

Although PII signal transduction proteins have been described in bacteria, archaea and higher plants, no PII homolog has so far been characterized in green algae. In the unicellular green alga Chlamydomonas reinhardtii, the PII protein is encoded by a single nuclear gene CrGLB1. The C. reinhardtii PII (CrPII) was cloned and overexpressed with a C-terminal-fused Strep-tag II peptide. Consistent with the presence of key conserved residues necessary for trimer formation, gel filtration showed the oligomeric structure of C. reinhardtii to be a homotrimer. Under the studied culture conditions, CrPII appears not to be modified by phosphorylation. Here we show that like its plant PII homologs, the CrPII protein is localized in the chloroplast. Although the CrGLB1 transcript level increased in response to dark-light shift and nitrogen depletion, the level of mature CrPII protein did not change accordingly. Changes in the level of CrGLB1 mRNA were independent of gametogenesis. Characterization of PII in the green alga C. reinhardtii provides a framework for a more complete understanding of the function of this highly conserved signaling protein.