Carboxysomes are an essential part of the cyanobacterial CO2-concentrating mechanism, consisting of a protein shell and an interior of Rubisco. The beta-carboxysome shell protein CcmM forms two peptides via a proposed internal ribosomal entry site (IRES) within the ccmM transcript in Synechococcus PCC7942. The abundant short form (35 kD, M35) consists of Rubisco small subunit-like repeats and binds Rubisco. The lower abundance long form (58 kD, M58) also contains a gamma-carbonic anhydrase-like domain, which binds the carboxysomal carbonic anhydrase, CcaA. We examined whether these CcmM forms arise via an IRES or by other means. Mutations of a putative internal start codon (GTG) and Shine-Dalgarno sequence within ccmM, along with a gene coding for M35 alone, were examined in the high-CO2-requiring (HCR) carboxysomeless mutant, DeltaccmM. Expression of wild-type ccmM in DeltaccmM restored the wild-type phenotype, while mutation of putative start and Shine-Dalgarno sequences led to as much as 20-fold reduction in M35 content with no recovery from HCR phenotype. These cells also contained small electron-dense structures. Cells producing little or no M58, but sufficient M35, were found to contain large electron-dense structures, no CcaA, and had a HCR phenotype. Large subcellular aggregates can therefore form in the absence of M58, suggesting a role for M35 in internal carboxysome Rubisco packing. The results confirm that M35 is independently translated via an IRES within ccmM. Importantly, the data reveal that functional carboxysomes require both M35 and M58 in sufficient quantities and with a minimum stoichiometry of close to 1:1.