Rebordinos L, Cross I, Merlo A
Area de Genética, Facultad de Ciencias del Mar y Ambientales, CEI-Mar, Universidad de Cádiz, Puerto Real, Spain.
Cytogenet Genome Res. 2013;141(2-3):103-13. doi: 10.1159/000354871. Epub 2013 Sep 24.
The 5S ribosomal DNA (rDNA) consists of one transcriptional unit of about 120 base pairs, which is separated from the next unit by a non-transcribed spacer (NTS). The coding sequence and the NTS together form a repeat unit which can be found in hundreds to thousands of copies tandemly repeated in the genomes. The NTS regions seem to be subject to rapid evolution. The first general model of evolution of these multigene families was referred to as divergent evolution, based on studies using hemoglobin and myoglobin as model systems. Later studies showed that nucleotide sequences of different multigene family members are more closely related within species than between species. This observation led to a new model of multigene family evolution, termed concerted evolution. Another model of evolution, named the birth-and-death model, has been found to be more suitable to explain the long-term evolution of these multigene families. According to this model, new genes originate by successive duplications, and these new genes are either maintained for a long time or are lost, or else degenerate into pseudogenes. In this review we describe different sources of variability in the 5S rDNA genes observed in several distinct fish species. This variability is mainly referred to NTSs and includes the presence of other multigene families (mainly LINEs, SINEs, non-LTR retrotransposons, and U snRNA families). Different types of microsatellites have also been found to contribute to the increase of variability in this region. Our recent results suggest that horizontal transfer contributes to the increase of diversity in the NTSs of some species. Variability in the 5S rDNA coding region affecting the stability of the structure, but without effects on the function of the 5S rRNA, is also described. Retrotransposons seem to be responsible for the high dynamism of 5S rDNA, while microsatellites acting as recombination hot spots could stabilize a wide variety of unusual DNA structures, affecting DNA replication and enhancing or decreasing promoter activity in gene expression. The relationship between the high variability found at molecular level and the low variability found at chromosomal level is also discussed.
5S核糖体DNA（rDNA）由一个约120个碱基对的转录单元组成，相邻单元之间由非转录间隔区（NTS）隔开。编码序列和NTS共同构成一个重复单元，在基因组中可以发现数百到数千个拷贝串联重复。NTS区域似乎经历快速进化。基于以血红蛋白和肌红蛋白为模型系统的研究，这些多基因家族进化的第一个通用模型被称为分歧进化。后来的研究表明，不同多基因家族成员的核苷酸序列在物种内比在物种间更为密切相关。这一观察结果导致了多基因家族进化的一个新模型，即协同进化。另一种进化模型，称为生死模型，已被发现更适合解释这些多基因家族的长期进化。根据该模型，新基因通过连续复制产生，这些新基因要么长期保留，要么丢失，要么退化为假基因。在这篇综述中，我们描述了在几种不同鱼类物种中观察到的5S rDNA基因变异的不同来源。这种变异主要指NTS，包括其他多基因家族（主要是LINEs、SINEs、非LTR逆转座子和U snRNA家族）的存在。还发现不同类型的微卫星有助于增加该区域的变异性。我们最近的结果表明，水平转移有助于增加某些物种NTS中的多样性。还描述了5S rDNA编码区影响结构稳定性但不影响5S rRNA功能的变异。逆转座子似乎是5S rDNA高动态性的原因，而作为重组热点的微卫星可以稳定多种异常DNA结构，影响DNA复制并增强或降低基因表达中的启动子活性。还讨论了在分子水平发现的高变异性与在染色体水平发现的低变异性之间的关系。
