University of Hawaii, Department of Microbiology, Honolulu, HI 96822, USA.
Mol Microbiol. 2012 Feb;83(4):682-93. doi: 10.1111/j.1365-2958.2011.07949.x. Epub 2012 Jan 4.
Nitrogen-fixing heterocysts are arranged in a periodic pattern on filaments of the cyanobacterium Anabaena sp. strain PCC 7120 under conditions of limiting combined nitrogen. Patterning requires two inhibitors of heterocyst differentiation, PatS and HetN, which work at different stages of differentiation by laterally suppressing levels of an activator of differentiation, HetR, in cells adjacent to source cells. Here we show that the RGSGR sequence in the 287-amino-acid HetN protein, which is shared by PatS, is critical for patterning. Conservative substitutions in any of the five amino acids lowered the extent to which HetN inhibited differentiation when overproduced and altered the pattern of heterocysts in filaments with an otherwise wild-type genetic background. Conversely, substitution of amino acids comprising the putative catalytic triad of this predicted reductase had no effect on inhibition or patterning. Deletion of putative domains of HetN suggested that the RGSGR motif is the primary component of HetN required for both its inhibitory and patterning activity, and that localization to the cell envelope is not required for patterning of heterocysts. The intercellular signalling proteins PatS and HetN use the same amino acid motif to regulate different stages of heterocyst patterning.
固氮异形胞在限氮条件下呈周期性排列在鱼腥藻 PCC 7120 的藻丝上。模式形成需要两种异形胞分化抑制剂 PatS 和 HetN，它们通过侧向抑制分化激活剂 HetR 在源细胞相邻细胞中的水平在分化的不同阶段发挥作用。在这里，我们表明，HetN 蛋白中 287 个氨基酸的 RGSGR 序列与 PatS 共享，对于模式形成至关重要。如果过表达，HetN 抑制分化的程度降低，并且在具有其他野生型遗传背景的藻丝中改变异形胞的模式，这五个氨基酸中的保守取代。相反，构成该预测还原酶的假定催化三联体的氨基酸取代对抑制或模式形成没有影响。HetN 的假定结构域缺失表明，RGSGR 基序是 HetN 发挥其抑制和模式形成活性所必需的主要成分，并且细胞包膜的定位对于异形胞的模式形成不是必需的。细胞间信号蛋白 PatS 和 HetN 使用相同的氨基酸基序来调节异形胞模式形成的不同阶段。
