Fig. 3.

(A) Representative electron micrographs of the wallless Chlamydomonas RbcS deletion mutant T60-3 (∆RbcS-host), complemented with Chlamydomonas (wild type), spinach (spinach hybrid), or Chlamydomonas/spinach chimeric RbcS constructs (helix AB and reciprocal). When the two SSU helices are encoded by the native Chlamydomonas sequence, a pyrenoid is formed (as in wild type and helix AB), irrespective of the nature of the rest of the SSU. However, when the two SSU helices are encoded by the spinach sequence, Rubisco does not aggregate into a pyrenoid, even when the rest of the SSU is Chlamydomonas-like (as in reciprocal). A phenotype of thylakoid hyperstacking was generally observed in the pyrenoidless strains (spinach hybrid and reciprocal; Fig. S4). Abbreviations: Cy, cytosol; Nu, nucleus; Cp, chloroplast; Py, pyrenoid; Stsh, starch sheath; Mt, mitochondrion; St, stromal starch grain; Pg, plastoglobule. (Scale bar, 500 nm.) (B) Proportional cross-sectional area of the pyrenoid in wild type (black columns) and helix AB (white columns). The area occupied by the “restored” pyrenoid in helix AB is not statistically different from wild type (unpaired t test, P = 0.143). (C) Detail of representative wild-type and helix AB pyrenoids probed with an antibody against wheat Rubisco. The density of pyrenoid labeling relative to the rest of the chloroplast was similar in both strains (unpaired t test, P = 0.077). Note that the starch sheath is interrupted by gaps allowing the passage of thylakoids.