Callahan S M, Buikema W J
Department of Molecular Genetics and Cell Biology, University of Chicago, 920 East 58th Street, Chicago, IL 60637 USA.
Mol Microbiol. 2001 May;40(4):941-50. doi: 10.1046/j.1365-2958.2001.02437.x.
The gene hetN encodes a putative oxidoreductase that is known to suppress heterocyst differentiation when present on a multicopy plasmid in Anabaena sp. PCC 7120. To mimic the hetN null phenotype and to examine where HetN acts in the regulatory cascade that controls heterocyst differentiation, we replaced the native chromosomal hetN promoter with the copper-inducible petE promoter. In the presence of copper, heterocyst formation was suppressed in undifferentiated filaments. When hetN expression was turned off by transferring cells to media lacking copper, the filaments initially displayed the wild-type pattern of single heterocysts but, 48 h after the induction of heterocyst formation, a pattern of multiple contiguous heterocysts predominated. Suppression of heterocyst formation by HetN appears to occur both upstream and downstream of the positive regulator HetR: overexpression of hetN in undifferentiated filaments prevents the wild-type pattern of hetR expression as well as the multiheterocyst phenotype normally observed when hetR is expressed from an inducible promoter. Green fluorescent protein fusions show that the expression of hetN in wild-type filaments normally occurs primarily in heterocysts. We propose that HetN is normally involved in the maintenance of heterocyst spacing after the initial heterocyst pattern has been established, but ectopic expression of hetN can also block the initial establishment of the pattern.
基因hetN编码一种假定的氧化还原酶，已知当该基因存在于鱼腥藻PCC 7120的多拷贝质粒上时，可抑制异形胞分化。为模拟hetN缺失表型并研究HetN在控制异形胞分化的调控级联反应中的作用位点，我们用铜诱导型petE启动子替换了天然染色体hetN启动子。在有铜存在的情况下，未分化丝状体中的异形胞形成受到抑制。当通过将细胞转移到不含铜的培养基中关闭hetN表达时，丝状体最初呈现单个异形胞的野生型模式，但在诱导异形胞形成48小时后，多个连续异形胞的模式占主导。HetN对异形胞形成的抑制似乎发生在正调控因子HetR的上游和下游：在未分化丝状体中hetN的过表达会阻止hetR表达的野生型模式以及当hetR从诱导型启动子表达时通常观察到的多异形胞表型。绿色荧光蛋白融合实验表明，野生型丝状体中hetN的表达通常主要发生在异形胞中。我们提出，在初始异形胞模式建立后，HetN通常参与异形胞间距的维持，但hetN的异位表达也可阻断该模式的初始建立。
