One gene, two proteins: coordinated production of a copper chaperone by differential transcript formation and translational frameshifting in Escherichia coli

Programmed ribosomal frameshifting (PRF) is a translational anomaly causing the ribosome to shift into an alternative reading frame. PRFs are common in viral genomes, using a single nucleotide sequence to code for two proteins in overlapping frames. In bacteria and eukaryota, PRFs are less frequent. We report on a PRF in the copper detoxification system of Escherichia coli where a metallochaperone is generated out of the first 69 amino acids and a Cterminal out-of-frame glycine of the gene copA. copA besides codes for the P-1B-ATPase CopA, a membrane-integral protein and principal interaction target of the chaperone. To enhance the production of the frameshift-generated cytosolic copper binding protein a truncated transcript is produced from the monocistronic copA gene. This shorter transcript is essential for producing sufficient amounts of the chaperone to support the membrane pump. The findings close the gap in our understanding of the molecular physiology of cytoplasmic copper transport in E. coli, revealing that a chaperone-like entity is required for full functionality of the P-1B-ATPase copper pump. We, moreover, demonstrate that the primary transcriptional response to copper results in formation of the small transcript and concurrently, the metallochaperone plays a key role in resistance against copper shock.