Chlamydomonas reinhardtii mutants without ribulose-1,5-bisphosphate carboxylase-oxygenase lack a detectable pyrenoid
The pyrenoid is a prominent proteinaceous structure found in the stroma of the chloroplast in unicellular eukaryotic algae, most multicellular algae, and some hornworts. The most prominent protein in the pyrenoid is the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). We have investigated whether the pyrenoid is present in strains of Chlamydomonas reinhardtii Dangeard containing mutations in the chloroplast rbcL gene. The mutants examined include a nonsense mutant lacking Rubisco, 18-7G, a missense mutant with an inactive Rubisco, 10-6C, and a temperature-sensitive mutant, 68-4PP, which contains Rubisco at room temperature but lacks the protein at 35°C. Normally, each C. reinhardtii cell has one chloroplast containing one large pyrenoid. In the nonsense mutant and 68-4PP at the non-permissive temperature no pyrenoid was observed. In the other strains, even those with an inactive Rubisco, the pyrenoid appeared normal. These results indicate that the presence of the Rubisco protein is necessary for the formation of a normal pyrenoid in C. reinhardtii. It is also clear that the Rubisco does not have to be active for normal pyrenoid formation, as strains 10-C and F-60 had morphologically normal pyrenoids.