Ram E V S Raghu, Naik Rangeetha, Ganguli Munia, Habib Saman
Division of Molecular and Structural Biology, Central Drug Research Institute, Lucknow-226 001, India.
Nucleic Acids Res. 2008 Sep;36(15):5061-73. doi: 10.1093/nar/gkn483. Epub 2008 Jul 28.
Apicomplexans, including the pathogens Plasmodium and Toxoplasma, carry a nonphotosynthetic plastid of secondary endosymbiotic origin called the apicoplast. The P. falciparum apicoplast contains a 35 kb, circular DNA genome with limited coding capacity that lacks genes encoding proteins for DNA organization and replication. We report identification of a nuclear-encoded bacterial histone-like protein (PfHU) involved in DNA compaction in the apicoplast. PfHU is associated with apicoplast DNA and is expressed throughout the parasite's intra-erythocytic cycle. The protein binds DNA in a sequence nonspecific manner with a minimum binding site length of approximately 27 bp and a K(d) of approximately 63 nM and displays a preference for supercoiled DNA. PfHU is capable of condensing Escherichia coli nucleoids in vivo indicating its role in DNA compaction. The unique 42 aa C-terminal extension of PfHU influences its DNA condensation properties. In contrast to bacterial HUs that bend DNA, PfHU promotes concatenation of linear DNA and inhibits DNA circularization. Atomic Force Microscopic study of PfHU-DNA complexes shows protein concentration-dependent DNA stiffening, intermolecular bundling and formation of DNA bridges followed by assembly of condensed DNA networks. Our results provide the first functional characterization of an apicomplexan HU protein and provide additional evidence for red algal ancestry of the apicoplast.
顶复门原虫，包括疟原虫和弓形虫等病原体，携带着一种起源于次生内共生的非光合质体，称为顶质体。恶性疟原虫的顶质体含有一个35 kb的环状DNA基因组，其编码能力有限，缺乏编码DNA组织和复制相关蛋白质的基因。我们报告了一种参与顶质体DNA压缩的核编码细菌组蛋白样蛋白（PfHU）的鉴定。PfHU与顶质体DNA相关，并在寄生虫的红细胞内周期中全程表达。该蛋白以序列非特异性方式结合DNA，最小结合位点长度约为27 bp，解离常数（K(d)）约为63 nM，且对超螺旋DNA表现出偏好。PfHU能够在体内凝聚大肠杆菌类核，表明其在DNA压缩中的作用。PfHU独特的42个氨基酸的C末端延伸影响其DNA凝聚特性。与能使DNA弯曲的细菌HU不同，PfHU促进线性DNA的连环化并抑制DNA环化。对PfHU-DNA复合物的原子力显微镜研究表明，蛋白质浓度依赖性的DNA变硬、分子间成束以及DNA桥的形成，随后是凝聚的DNA网络的组装。我们的结果首次对顶复门原虫的HU蛋白进行了功能表征，并为顶质体的红藻起源提供了额外证据。
