Abstract
Synechocystis PCC 6803 contains four genes encoding polypeptides with sequence features of CPx-type ATPases, two of which are now designated pacS and ctaA. We show that CtaA and PacS (but not the related transporters, ZiaA or CoaT) facilitate switching to the use of copper (in plastocyanin) as an alternative to iron (in cytochrome c(6)) for the carriage of electrons within the thylakoid lumen. Disruption of pacS reduced copper tolerance but enhanced silver tolerance, and pacS-mediated restoration of copper tolerance was used to select transformants. Disruption of ctaA caused no change in copper tolerance but reduced the amount of copper cell(-1). In cultures supplemented with 0.2 microm copper, photooxidation of cytochrome c(6) (PetJ) was depressed in wild-type cells but remained elevated in both Synechocystis PCC 6803(ctaA) and Synechocystis PCC 6803(pacS). Conversely, plastocyanin transcripts (petE) were less abundant in both mutants at this [copper]. Synechocystis PCC 6803(ctaA) and Synechocystis PCC 6803(pacS) showed increased iron dependence with impaired growth in deferoxamine mesylate (iron chelator)-containing media. Double mutants also deficient in cytochrome c(6), Synechocystis PCC 6803(petJ,ctaA) and Synechocystis PCC 6803(petJ,pacS), were viable, but the former had increased copper dependence with severely impaired growth in the presence of bathocuproinedisulfonic acid (copper chelator). Analogous transporters are likely to supply copper to plastocyanin in chloroplasts.
Highlights
Cyanobacteria contain internal thylakoid membranes where oxygen-evolving photosynthetic electron transport occurs
Synechocystis PCC 6803(pacS) is unable to grow in BG-11-C medium containing 1 M copper and shows some inhibition of growth in medium containing 0.3 M copper
Restoration of tolerance to 3 M copper was used as a selectable marker to identify mutants of Synechocystis PCC 6803(pacS) in which pacS had reintegrated into the chromosome by homologous recombination following incubation of cells with the corresponding DNA
Summary
Both pacS and ctaA in Synechocystis PCC 6803 is negative with respect to the photooxidation of cytochrome c6 and positive toward the accumulation of plastocyanin, petE, transcripts in copper-containing medium. Both of these transporters contribute toward the substitution of copper (in place of iron) for photosynthetic electron transport in the thylakoid, consistent with inward copper transport by both. Deletion of ctaA impairs cellular copper accumulation. Deletion of pacS confers copper sensitivity but silver resistance, which is interpreted in the context of metal ion sequestration within thylakoids. Structural features that confer metal specificity are considered
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
