Dissection of Nutrient Deprivation Responses in Cyanobacteria and the Degradation of the Light-Harvesting Complex

9727836格罗斯曼藻胆体是一种多蛋白质复合体，可以获取蓝藻和真核红藻光合作用所需的大部分光能。当蓝藻缺乏必需的常量营养素时，生物体会经历一系列反应，其中一些是针对限制营养的(例如，帮助获得限制营养的运输系统的表达增加)，而另一些是普遍的，发生在一些不同的胁迫条件下(例如，藻胆体的降解)。关于控制一般和具体反应的调控因素的知识很少；它们可能有任何交叉的控制点。为了分离在一般反应的调节方面存在缺陷的突变体，使用可视化屏幕来鉴定在缺硫期间不能降解其藻胆体的菌株。许多突变体都有一个名为nblA(非漂白)的基因损伤，该基因编码一种59个氨基酸的多肽，这是藻胆体降解所必需的。另外6个在nblA中没有损伤的非漂白突变体已经被鉴定。其中四个突变体在营养限制期间不表达nblA，其余两个在导致蛋白质降解的途径中位于nblA多肽的下游。所有不表达nblA的突变体都得到了一个基因的补充，该基因编码了一个典型的双组分调控系统的反应调节因子。正在使用四个不同的筛选来分离调节一般反应和硫胁迫特定反应的额外突变体。其中一个筛选还可以分离出在导致藻胆体降解的途径中NblA下游元件有缺陷的其他突变体；这个筛选可以确定与NblA相互作用的蛋白质和导致多亚单位捕光复合体快速降解的机制(S)。阐明NblA在漂白过程中的作用非常重要，因为虽然人们知道蛋白质分解对于消除变性、不正确折叠和未组装的蛋白质以及控制发育过程和调节级联反应是多么关键，但对支配蛋白质分解的机制和特定蛋白质复合体的降解方式知之甚少。

9727836 Grossman Phycobilisomes are multiprotein complexes that harvest most of the light energy that is used to drive photosynthesis in cyanobacteria and eukaryotic red algae. When cyanobacteria are deprived of an essential macronutrient, the organism undergoes a suite of responses, some of which are specific to the limiting nutrient (e.g. elevated expression of transport systems that help in the acquisition of the limiting nutrient), while others are general and occur during a number of different stress conditions (e.g. the degradation of phycobilisomes). There is little knowledge concerning the regulatory elements that control the general and specific responses; they may have any intersecting control points. To isolate mutants with defects in the regulation of the general responses, a visual screen to identify strains unable to degrade their phycobilisomes during sulfur deprivation was used. Many of the mutants had a lesion in a gene, designated nblA (for nonbleaching), that encodes a polypeptide of 59 amino acids which is essential for phycobilisome degradation. Six additional nonbleaching mutants which do not have lesions in nblA have been identified. Four of the mutants do not express nblA during nutrient limitation and the remaining two are downstream of the NblA polypeptide in the pathway leading to protein degradation. All of the mutants that do not express nblA are complemented by a gene that encodes a response regulator typical of two-component regulatory systems. Four distinct screens to isolate additional mutants in the regulation of the general responses and the sulfur stress-specific responses are being used. One of the screens would also allow the isolation of additional mutants defective in elements downstream of NblA in the pathway leading to phycobilisome degradation; this screen could identify the proteins with which NblA interacts and the mechanism(s) that lead to the rapid degradation of the multisubunit light-harvesting complex. It is important to elucidate the function of NblA in the bleaching process since, while it is understood how critical proteolysis is for the eliminating denatured, improperly folded and unassembled proteins and for controlling developmental processes and regulatory cascades, little is known about the mechanisms that govern proteolysis and the ways in which specific protein complexes are targeted for degradation.