Lack of the Rhesus protein Rh1 impairs growth of the green alga Chlamydomonas reinhardtii at high CO2

Although Rhesus (Rh) proteins are best known as antigens on human red blood cells, they are not restricted to red cells or to mammals, and hence their primary biochemical functions can be studied in more tractable organisms. We previously established that the Rh1 protein of the green alga Chlamydomonas reinhardtii is highly expressed in cultures bubbled with air containing high CO2 (3%), conditions under which Chlamydomonas grows rapidly. By RNA interference, we have now obtained Chlamydomonas rh mutants (epigenetic), which are among the first in nonhuman cells. These mutants have essentially no mRNA or protein for RH1 and grow slowly at high CO2, apparently because they fail to equilibrate this gas rapidly. They grow as well as their parental strain in air and on acetate plus air. However, during growth on acetate, rh1 mutants fail to express three proteins that are known to be down-regulated by high CO2: periplasmic and mitochondrial carbonic anhydrases and a chloroplast envelope protein. This effect is parsimoniously rationalized if the small amounts of Rh1 protein present in acetate-grown cells of the parental strain facilitate leakage of CO2 generated internally. Together, these results support our hypothesis that the Rh1 protein is a bidirectional channel for the gas CO2. Our previous studies in a variety of organisms indicate that the only other members of the Rh superfamily, the ammonium/methylammonium transport proteins, are bidirectional channels for the gas NH3. Physiologically, both types of gas channels can apparently function in acquisition of nutrients and/or waste disposal.