Photosynthesis in cyanobacteria, green algae, and basal land plants is protected against excess reducing pressure on the photosynthetic chain by flavodiiron proteins (FLV) that dissipate photosynthetic electrons by reducing O₂. In these organisms, the genes encoding FLV are always conserved in the form of a pair of two-type isozymes (FLVA and FLVB) that are believed to function in O₂ photo-reduction as a heterodimer. While coral symbionts (dinoflagellates of the family Symbiodiniaceae) are the only algae to harbor FLV in photosynthetic red plastid lineage, only one gene is found in transcriptomes and its role and activity remain unknown. Here, we characterized the FLV genes in Symbiodiniaceae and found that its coding region is composed of tandemly repeated FLV sequences. By measuring the O₂-dependent electron flow and P700 oxidation, we suggest that this atypical FLV is active in vivo. Based on the amino-acid sequence alignment and the phylogenetic analysis, we conclude that in coral symbionts, the gene pair for FLVA and FLVB have been fused to construct one coding region for a hybrid enzyme, which presumably occurred when or after both genes were inherited from basal green algae to the dinoflagellate. Immunodetection suggested the FLV polypeptide to be cleaved by a post-translational mechanism, adding it to the rare cases of polycistronic genes in eukaryotes. Our results demonstrate that FLV are active in coral symbionts with genomic arrangement that is unique to these species. The implication of these unique features on their symbiotic living environment is discussed.

_{黄酮二铁蛋白 (FLV) 通过还原 O 2}来耗散光合电子，从而保护蓝藻、绿藻和基础陆地植物的光合作用免受光合链上的过度还原压力。在这些生物体中，编码FLV的基因总是以一对被认为在O ₂光还原中作为异二聚体起作用的两种同工酶（FLVA和FLVB）的形式保守。虽然珊瑚共生体（共生藻科的甲藻）是光合作用红色质体谱系中唯一携带 FLV 的藻类，但在转录组中仅发现一个基因，其作用和活性仍然未知。在这里，我们描述了FLV共生藻科基因，发现其编码区由串联重复的FLV序列组成。通过测量 O ₂依赖电子流和 P700 氧化，我们认为这种非典型 FLV 在体内是活跃的。基于氨基酸序列比对和系统发育分析，我们得出结论，在珊瑚共生体中，FLVA 和 FLVB 的基因对已经融合，构建了一个杂交酶的编码区，这可能发生在两个基因遗传时或之后从基础绿藻到甲藻。免疫检测表明 FLV 多肽通过翻译后机制被切割，将其添加到真核生物中罕见的多顺反子基因病例中。我们的研究结果表明，FLV 在珊瑚共生体中具有活性，其基因组排列对这些物种来说是独一无二的。讨论了这些独特特征对其共生生活环境的影响。