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

藻胆体是一种多蛋白复合物，它能收集蓝藻和真核红藻中用于驱动光合作用的大部分光能。当蓝藻被剥夺一种必需的宏量营养素时，生物体会经历一系列反应，其中一些是特定于限制性营养物质的（例如，帮助获得限制性营养物质的运输系统的表达升高），而另一些则是一般的，发生在许多不同的应激条件下（例如，藻胆异构体的降解）。人们对控制一般和特定反应的监管要素知之甚少；它们可以有任何相交的控制点。为了分离一般反应调节缺陷的突变体，使用视觉筛选来识别在硫剥夺期间无法降解其藻胆体的菌株。许多突变体在一个被命名为nblA（非漂白）的基因上有损伤，该基因编码一种由59个氨基酸组成的多肽，对藻胆酶体的降解至关重要。另外六种非漂白突变体在nblA中没有病变。其中四个突变体在营养限制期间不表达nblA，其余两个突变体位于导致蛋白质降解途径中nblA多肽的下游。所有不表达nblA的突变体都有一个编码双组分调节系统典型反应调节因子的基因补充。目前正在使用四种不同的筛选方法来分离一般反应和硫胁迫特异性反应调节中的其他突变体。其中一种筛选方法还可以分离出NblA下游导致藻胆酶体降解途径中存在缺陷的其他突变体；该筛选可以识别与NblA相互作用的蛋白质以及导致多亚基光收集复合物快速降解的机制。阐明NblA在漂白过程中的功能是很重要的，因为虽然我们了解蛋白质水解对于消除变性、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的关键作用，但我们对蛋白质水解的机制和特定蛋白质复合物被降解的方式知之甚少。

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.