Functional CTR-type Cu(I) transporters in an oceanic diatom

Copper concentration is so low in some remote parts of the sea it limits phytoplankton growth, but may be high enough in coastal and estuarine regions to be toxic. Acclimation to variations in Cu concentration thus requires a tightly regulated Cu transport system to help maintain Cu homeostasis. In marine species, the molecular mechanisms of Cu transport are not known. We studied Cu-responsive genes and uptake in Thalassiosira oceanica at environmentally relevant Cu concentrations varying between 0.012 and 12 900 pmol Cu' l^-1 . Copper uptake rate assessed at high Cu concentration was three-fold faster in Cu-limited than in Cu-replete cells, confirming the existence of an inducible uptake pathway in this diatom. Four putative CTR-type Cu transporters (ToCTR1, ToCTR2, ToCTR3a and ToCTR3b) identified in the transcriptome shared conserved features with known high-affinity Cu(I) transporters. Expression of the CTR genes was upregulated as Cu concentration declined and cells maintained maximum rates of growth. Further decreases in Cu led to decreased growth rate and increased abundance of ToCTR3a/b. Both ToCTR3a and 3b restored growth of a Cu transport mutant, Saccharomyces cerevisiae ctr1Δctr3Δ, in Cu-deficient medium and increased the uptake rates of Cu(I) and Cu(II). Thus, ToCTR3a/3b is a high-affinity Cu(I) transporter that, in conjunction with the other ToCTRs, may enable T. oceanica to survive in Cu-deplete ocean environments and respond to natural variation in Cu availability.