The molecular basis of the most red-shifted allophycocyanin discovered to date.

Allophycocyanin (APC) protein subunits responsible for red-shifted phycobilisomes are induced under far-red light conditions. The ApcB2 (encoded by gene XM38_020890) and ApcD4 (encoded by gene XM38_020900) in H. hongdechloris are paralogous APC subunits encoded in the termed low-light photoacclimation (LoLiP) gene cluster, which are only detected from cells grown under far-red light (FRL) conditions. We examined the function of these allophycocyanin subunits using heterogeneous recombinant E. coli systems. The recombinant chromophorylated ApcB2 showed absorptionpeaking at 618 nm and fluorescence peaking at 642 nm and the chromophorylated ApcD4 demonstrated two absorption peaks of 618 and 676 nm and fluorescence peaks of 625 and 698 nm, respectively. Interestingly, the heterodimer of ApcB2/ApcD4 demonstrated even further FRL absorption of 728 nm and fluorescence emission peaking at 742 nm. Using ΔApcB2-W75T to replace ApcB2 for APC ab heterodimeric formation, the red-shifted absorption at 728 nm disappeared, suggesting that Trp75 of ApcB2 is essential for the heterodimer maintaining the red-shifted 728 nm spectroscopic feature. The extremely red-shifted spectroscopic properties of ApcD4/ApcB2 complexes reveal the strain-specific diversity of FRL-phycobilisomes and advance our understanding of remodelled light-harvesting complexes that capture FRL. In H. hongdechloris, besides the well-known Far-red light Photoacclimation (FaRLiP) gene cluster, the APC αβ heterodimer of ApcB2/ApcD4 from LoLiP gene cluster likely functions as the terminal emitter of red-shifted phycobilisomes for chlorophyll f-binding protein complexes. The recombinant, red-shifted APC αβ heterodimer offers a potential new class of fluorescence labels in the near-infrared spectral region.