Department of Biology, University of the Pacific, Stockton, California, USA.
Department of Biology, University of the Pacific, Stockton, California, USA
J Bacteriol. 2020 Feb 11;202(5). doi: 10.1128/JB.00675-19.
Filamentous, heterocyst-forming cyanobacteria belonging to taxonomic subsections IV and V are developmentally complex multicellular organisms capable of differentiating an array of cell and filament types, including motile hormogonia. Hormogonia exhibit gliding motility that facilitates dispersal, phototaxis, and the establishment of nitrogen-fixing symbioses. The gene regulatory network (GRN) governing hormogonium development involves a hierarchical sigma factor cascade, but the factors governing the activation of this cascade are currently undefined. Here, using a forward genetic approach, we identified , a gene encoding a putative hybrid histidine kinase that functions upstream of the sigma factor cascade. The deletion of produced nonmotile filaments that failed to display hormogonium morphology or accumulate hormogonium-specific proteins or polysaccharide. Targeted transcriptional analyses using reverse transcription-quantitative PCR (RT-qPCR) demonstrated that hormogonium-specific genes both within and outside the sigma factor cascade are drastically downregulated in the absence of and that may be subject to indirect, positive autoregulation via and Orthologs of HrmK are ubiquitous among, and exclusive to, heterocyst-forming cyanobacteria. Collectively, these results indicate that functions upstream of the sigma factor cascade to initiate hormogonium development, likely by modulating the phosphorylation state of an unknown protein that may serve as the master regulator of hormogonium development in heterocyst-forming cyanobacteria. Filamentous cyanobacteria are morphologically complex, with several representative species amenable to routine genetic manipulation, making them excellent model organisms for the study of development. Furthermore, two of the developmental alternatives, nitrogen-fixing heterocysts and motile hormogonia, are essential to establish nitrogen-fixing symbioses with plant partners. These symbioses are integral to global nitrogen cycles and could be artificially recreated with crop plants to serve as biofertilizers, but to achieve this goal, detailed understanding and manipulation of the hormogonium and heterocyst gene regulatory networks may be necessary. Here, using the model organism , we identify a previously uncharacterized hybrid histidine kinase that is confined to heterocyst-forming cyanobacteria as the earliest known participant in hormogonium development.
丝状异形胞形成的蓝藻属于分类亚节 IV 和 V，是具有复杂发育能力的多细胞生物，能够分化出一系列细胞和丝状类型，包括游动的类菌毛。类菌毛表现出滑行运动，有助于分散、趋光性和固氮共生体的建立。调节类菌毛发育的基因调控网络（GRN）涉及分层σ因子级联，但目前尚不清楚激活此级联的因素。在这里，我们使用正向遗传方法鉴定了一个编码假定杂合组氨酸激酶的基因，该基因位于σ因子级联的上游。缺失 导致非运动性丝状体无法显示类菌毛形态或积累类菌毛特异性蛋白或多糖。使用反转录定量 PCR（RT-qPCR）进行靶向转录分析表明，在缺乏 的情况下，σ因子级联内外的类菌毛特异性基因均被严重下调，并且 可能通过 和 间接正自调控。HrmK 的同源物在异形胞形成的蓝藻中普遍存在，且是异形胞形成的蓝藻所特有的。总的来说，这些结果表明 在上游的 σ因子级联中起作用，以启动类菌毛的发育，可能通过调节未知蛋白的磷酸化状态来实现，该蛋白可能作为异形胞形成的蓝藻中类菌毛发育的主调控因子。丝状蓝藻形态复杂，有几个代表性物种可进行常规遗传操作，使其成为发育研究的优秀模式生物。此外，两种发育选择，固氮异形胞和游动的类菌毛，对于与植物伙伴建立固氮共生体至关重要。这些共生体是全球氮循环的组成部分，可以与作物植物一起人工重建，作为生物肥料，但要实现这一目标，可能需要详细了解和操纵类菌毛和异形胞基因调控网络。在这里，我们使用模式生物 鉴定了一个以前未被表征的杂合组氨酸激酶，它局限于异形胞形成的蓝藻中，是最早已知参与类菌毛发育的成员。
