Marinov Georgi K, Lynch Michael
Department of Biology, Indiana University, Bloomington, 47405, IN, United States.
Biol Direct. 2016 Apr 5;11(1):18. doi: 10.1186/s13062-016-0119-4.
Nucleomorphs, the remnant nuclei of photosynthetic algae that have become endosymbionts to other eukaryotes, represent a unique example of convergent reductive genome evolution in eukaryotes, having evolved independently on two separate occasions in chlorarachniophytes and cryptophytes. The nucleomorphs of the two groups have evolved in a remarkably convergent manner, with numerous very similar features. Chief among them is the extreme reduction and compaction of nucleomorph genomes, with very small chromosomes and extremely short or even completely absent intergenic spaces. These characteristics pose a number of intriguing questions regarding the mechanisms of transcription and gene regulation in such a crowded genomic context, in particular in terms of the functioning of the histone code, which is common to almost all eukaryotes and plays a central role in chromatin biology.
This study examines the sequences of nucleomorph histone proteins in order to address these issues. Remarkably, all classical transcription- and repression-related components of the histone code seem to be missing from chlorarachniophyte nucleomorphs. Cryptophyte nucleomorph histones are generally more similar to the conventional eukaryotic state; however, they also display significant deviations from the typical histone code. Based on the analysis of specific components of the code, we discuss the state of chromatin and the transcriptional machinery in these nuclei.
The results presented here shed new light on the mechanisms of nucleomorph transcription and gene regulation and provide a foundation for future studies of nucleomorph chromatin and transcriptional biology.
核质体是光合藻类的残余细胞核，已成为其他真核生物的内共生体，代表了真核生物中趋同的还原性基因组进化的独特例子，在绿藻虫和隐藻中分别独立进化了两次。这两组的核质体以非常趋同的方式进化，具有许多非常相似的特征。其中最主要的是核质体基因组的极度缩减和压缩，染色体非常小，基因间间隔极短甚至完全没有。这些特征引发了许多关于在如此拥挤的基因组环境中转录和基因调控机制的有趣问题，特别是关于几乎所有真核生物共有的组蛋白密码的功能，其在染色质生物学中起着核心作用。
本研究检查了核质体组蛋白的序列以解决这些问题。值得注意的是，绿藻虫核质体中似乎缺少所有与组蛋白密码经典转录和抑制相关的成分。隐藻核质体组蛋白通常与传统的真核状态更相似；然而，它们也与典型的组蛋白密码存在显著偏差。基于对密码特定成分的分析，我们讨论了这些细胞核中染色质和转录机制的状态。
本文给出的结果为核质体转录和基因调控机制提供了新的见解，并为未来核质体染色质和转录生物学的研究奠定了基础。
