An RbcL-specific chaperone. The central cleft of the RbcX homodimer (RbcX2) binds the C-terminus of an RbcL monomer, stabilizing the C-terminus and probably preventing its reassociation with chaperonin GroEL-ES. At the same time the peripheral region of RbcX2 binds a second RbcL monomer, bridging the RbcL homodimers in the correct orientation. The RbcX2(2)-bound RbcL dimers then assemble into the RbcL8 core (RbcL8-(RbcX2)8). RbcS binding triggers the release of RbcX2. Colocalizes with RuBisCO throughout carboxysome formation, suggesting it plays a role in caboxysome biogenesis; may also be involved in RuBisCO holoenzyme stabilization or adjusting its packing in carboxysomes. Improves functional assembly of RuBisCO upon coexpression in E.coli with the large and small subunits of RuBisCO (rbcL and rbcS), however it is not essential in vivo (PubMed:17071623). Homodimer. Interacts with the exposed C-terminal peptide of RbcL via its central cleft, contacts a second RbcL monomer via its peripheral polar surface. Most protein is cytoplasmic, but some is in the carboxysome. Transcribed at very low levels under several growth conditions; in this paper protein was never detected (PubMed:17071623). Carboxysome size and components vary with growth conditions. When grown in ambient air at medium light (50 uE meter(-2) second(-1)) there are 40 units of this protein per carboxysome, which remain constant under all conditions tested (at protein level) (PubMed:31048338). The homodimer has 2 functional domains, a central cleft essential for production of soluble RbcL in which the RbcL peptide binds, and a polar surface which plays a role in correct RbcL subunit arrangement. Non-essential, no visible growth phenotype in ambient air or 2% CO(2), no change in RuBisCO assembly or activity (PubMed:17071623). Non-essential, has a reduced growth rate in 5% CO(2), about 2-fold higher amounts of RbcL accumulate. Cells have fewer, larger carboxysomes, their positioning is often abnormal (PubMed:30389782). A double raf1-rbcX deletion partially recovers carboxysome formation compared to the raf1 deletion alone (PubMed:32636267). Heterologous expression of 12 carboxysomal genes in E.coli (ccaA, ccmK2, ccmK3, ccmK4, ccmL, ccmM, ccmN, ccmO, ccmP, rbcL, rbcS, rbcX) leads to the formation of bodies that resemble carboxysomes, have densely packed paracrystalline arrays and RuBisCO activity. These structures open the door to generating carboxysomes in plant cells to increase their photosynthesis and productivity, as well as tailoring bacterial microcompartments to specific metabolic needs and molecule delivery. The absence of ccaA, ccmK3, ccmK4, ccmP and rbcX leads to less active RuBisCO. Unlike most cyanobacteria, in PCC 7942 this gene is not encoded in the rbcL-rbcS operon. Belongs to the RbcX family.