Bilin lyases are enzymes which ligate linear tetrapyrrole chromophores to specific cysteine residues on light harvesting proteins present in cyanobacteria and red algae. The lyases responsible for chromophorylating the light harvesting protein phycoerythrin (PE) have not been fully characterized. In this study, we explore the role of CpeT, a putative bilin lyase, in the biosynthesis of PE in the cyanobacterium Fremyella diplosiphon. Recombinant protein studies show that CpeT alone can bind phycoerythrobilin (PEB), but CpeZ, a chaperone-like protein, is needed in order to correctly and efficiently attach PEB to the β-subunit of PE. MS analyses of the recombinant β-subunit of PE coexpressed with CpeT and CpeZ show that PEB is attached at Cys-165. Purified phycobilisomes from a cpeT knockout mutant and wild type (WT) samples from F. diplosiphon were analyzed and compared. The cpeT mutant contained much less PE and more phycocyanin than WT cells grown under green light, conditions which should maximize the production of PE. In addition, Northern blot analyses showed that the cpeCDESTR operon mRNAs were upregulated while the cpeBcpeA mRNAs were downregulated in the cpeT mutant strain when compared with WT, suggesting that CpeT may also play a direct or indirect regulatory role in transcription of these operons or their mRNA stability, in addition to its role as a PEB lyase for Cys-165 on β-PE.

胆红素裂解酶是将线性四吡咯发色团连接到蓝细菌和红藻中存在的光收集蛋白上的特定半胱氨酸残基的酶。尚未完全鉴定负责使光收集蛋白藻红蛋白（PE）磷酸化的裂解酶。在这项研究中，我们探讨了推定的胆汁酸裂解酶CpeT在蓝藻弗雷米氏双歧杆菌PE的生物合成中的作用。重组蛋白研究表明，单独的CpeT可以结合藻红蛋白（PEB），但是需要CpeZ（一种伴侣蛋白）才能正确有效地将PEB连接到PE的β亚基上。与CpeT和CpeZ共表达的PE的重组β-亚基的MS分析表明，PEB附着在Cys-165上。从cpeT中纯化的藻胆体分析和比较了来自F. diplosiphon的基因敲除突变体和野生型（WT）样品。与在绿光下生长的WT细胞相比，cpeT突变体包含的PE少得多，藻蓝蛋白更多，这应该使PE的产量最大化。此外，Northern印迹分析显示，与WT相比，cpeT突变株中的cpeCDESTR操纵子mRNA上调，而cpeBcpeA mRNA则下调，这表明CpeT在这些操纵子或其mRNA的转录中也可能起直接或间接的调节作用。除了用作β-PE上Cys-165的PEB裂解酶外，还具有稳定性。