CCM systems are found in some eubacteria, mitochondria of protist and red algae. ORFs homologous to CcmB and CcmC were found on the mitochondrial genome of plants ( Pubmed : 7529870 ) , ( Pubmed : 7862087 ) , ( Pubmed : 8003696 ) , and this suggests that the missing gene encoding the ABC subunit was moved onto the chromosome. This hypothesis has been recently proven to be true in the case of Arabidopsis thaliana where a complete CCM system was found to be located on the nuclear genome ( Pubmed : 15381434 ) . A typical system is comprised of an ABC protein (CcmA) and 2 IM proteins (CcmB and CcmC). The actual organization of these systems is debated. It has been proposed that they form an heterodimeric complex made of CcmB CcmC and 2 copies of CcmA involved in the export of heme into the periplasmic space ( Pubmed : 9560218 ) . An alternate view proposes that CcmC is essential for heme biogenesis and that a homodimer made of 2 copies of CcmA CcmB constitutes a transporter of unknown substrate ( Pubmed : 10339610 ) .