Cyanobacterial sigma factor controls biofilm-promoting genes through intra- and intercellular pathways

Cyanobacteria frequently constitute integral components of microbial communities known as phototrophic biofilms, which are widespread in various environments and hold significant industrial relevance. Previous studies of the model cyanobacterium Synechococcus elongatus PCC 7942 revealed that its planktonic growth habit results from a biofilm-suppression mechanism that depends on an extracellular inhibitor, an observation that opens the door to investigating cyanobacterial intercellular communication. Here, we demonstrate that the RNA polymerase sigma factor SigF1, is required for this biofilm-suppression mechanism and suggest that sigF1-inactivation impairs secretion of the biofilm inhibitor. The S. elongatus paralog SigF2, however, is not involved in biofilm regulation. Comprehensive transcriptome analyses identified distinct regulons under the control of each of these sigma factors. Additional data indicate that SigF1 regulates biofilm through its involvement in transcriptional induction of genes that include those for the primary pilus subunit: sigF1 inactivation both prevents pilus assembly and abrogates secretion of the biofilm inhibitor. Consequently, expression is significantly upregulated for the ebfG-operon that encodes matrix components and the genes that encode the corresponding secretion system. Thus, this study uncovers a basic regulatory component of cyanobacterial intercellular communication, a field that is in its infancy. Elevated expression of biofilm-promoting genes in a sigF1 mutant supports an additional layer of regulation by SigF1 that operates via an intracellular mechanism. To uncover transcriptional changes associated with biofilm formation we compared the transcriptomes of WT and the biofilm-forming strains PilB::Tn5 and SigF1::Mu. In addition, SigF2 mutant was included in the analysis to define the transcriptional space of either one of these SigF factors.

蓝细菌（Cyanobacteria）通常是光营养生物膜（phototrophic biofilms）这一类微生物群落的核心组成部分，这类生物膜在各类环境中广泛分布，且具有重要的工业应用价值。此前针对模式蓝细菌长形聚球藻（Synechococcus elongatus）PCC 7942的研究显示，其浮游生长习性源自依赖胞外抑制剂的生物膜抑制机制，这一发现为探究蓝细菌细胞间通讯开辟了全新路径。本研究证实，RNA聚合酶σ因子SigF1是该生物膜抑制机制所必需的，并表明sigF1基因失活会削弱生物膜抑制剂的分泌。而长形聚球藻的旁系同源基因SigF2则不参与生物膜调控。全面的转录组（transcriptome）分析揭示了这两个σ因子各自调控的独特调节子（regulon）。补充数据表明，SigF1通过参与包括主要菌毛亚基编码基因在内的基因的转录诱导来调控生物膜：sigF1失活既能阻断菌毛组装，又能消除生物膜抑制剂的分泌。因此，编码基质组分的ebfG操纵子（ebfG-operon）以及编码对应分泌系统的基因的表达均显著上调。综上，本研究揭示了蓝细菌细胞间通讯这一新兴领域中的基础调控组分。sigF1突变体中生物膜促进基因的表达上调，进一步佐证了SigF1可通过胞内机制实现另一层面的调控。为探究与生物膜形成相关的转录组变化，本研究对比了野生型（WT）、生物膜形成菌株PilB::Tn5以及SigF1::Mu的转录组。此外，本研究还纳入了SigF2突变体，以明确这两个SigF因子各自的转录调控范围。