Description:
<jats:p>Various forms of <jats:italic>Bradyrhizobioum japonicum</jats:italic> cytochrome <jats:italic>c</jats:italic><jats:sub>550</jats:sub>, (the <jats:italic>cycA</jats:italic> gene product) were overexpressed in <jats:italic>Escherichia coli</jats:italic> cells grown under different conditions. Antibodies directed against a synthetic cytochrome <jats:italic>c</jats:italic><jats:sub>550</jats:sub> peptide were used as tools to detect both, apoprotein and holoprotein. Complete maturation of the apoprotein into its holo from with haem covalently bound to the polypeptide was observed only under anaerobic growth conditions and in <jats:italic>E. coli</jats:italic> K12 derivatives, whereas there binding did not occur in the <jats:italic>E. coli</jats:italic> EL21 host. When maturation was complete, holocytochrorme <jats:italic>c</jats:italic><jats:sub>550</jats:sub> was found exclusively in the periplasmic fraction. A <jats:italic>cycA</jats:italic> ‐expressing plasmid construct lacking the genetic information for the signal sequence produced apoprotein that wits rapidly degraded without further maturation. Mutations in the haem‐binding site resulted in products that were translocated through the cytoplasmic membrane, but apparently became degraded. Our results support the view that attachment of haem to the apoprotein is not a prerequisite for cleavage of the signal sequence and occurs on the periplasmic side of the membrane, subsequent to translocation of the apoprotein precursor.</jats:p>