Department of Microbiology, University of Hawaii, Honolulu, HI 96822, USA.
Mol Microbiol. 2010 Aug;77(3):562-74. doi: 10.1111/j.1365-2958.2010.07257.x. Epub 2010 Jun 9.
Differentiation of nitrogen-fixing cells, called heterocysts, by the cyanobacterium Anabaena sp. strain PCC 7120 requires HetR, which is considered the master regulator of heterocyst differentiation. In this study, ectopic expression of hetP from an inducible promoter was shown to partially bypass the need for hetR in heterocyst differentiation. A hetR-deletion strain with hetP expressed ectopically produced cells morphologically similar to heterocysts that produced exopolysaccharide and glycolipids specific to heterocysts, and nitrogenase activity was present under anaerobic conditions, as indicated by an acetylene reduction assay. Five potential transcription start points or transcript processing sites located at positions -727, -545, -208, -177 and -12 bp relative to the putative translational start site were identified in the promoter region of hetP. Relative levels of transcription and their location in filaments resembled that previously reported for HetR protein in various genetic backgrounds, and an inverted repeat sequence in the promoter of hetP was necessary for binding of HetR. Two of the three identified homologues of hetP found in PCC 7120 partially complemented a hetP-null mutant. Taken together, these results suggest that HetP functions directly downstream of HetR in the regulatory network responsible for heterocyst differentiation.
固氮细胞（称为异形胞）的分化由鱼腥藻 PCC 7120 菌株的 HetR 调控，HetR 被认为是异形胞分化的主调控因子。在本研究中，来自诱导型启动子的异位表达 hetP 显示可部分绕过异形胞分化中对 hetR 的需求。具有异位表达 hetP 的 hetR 缺失菌株产生的细胞在形态上类似于异形胞，可产生特定于异形胞的胞外多糖和糖脂，并且在乙炔还原测定中，厌氧条件下存在固氮酶活性。在 hetP 的启动子区域中鉴定了五个潜在的转录起始点或转录处理位点，它们相对于假定的翻译起始位点的位置分别为-727、-545、-208、-177 和-12 bp。转录的相对水平及其在丝状体中的位置与 HetR 蛋白在各种遗传背景下的先前报道相似，并且 hetP 启动子中的反向重复序列是 HetR 结合所必需的。在 PCC 7120 中发现的 hetP 的三个同源物中的两个部分互补了 hetP 缺失突变体。总之，这些结果表明 HetP 在负责异形胞分化的调控网络中直接位于 HetR 下游起作用。
