Buckley Katherine M, Smith L Courtney
George Washington University, Department of Biological Sciences, Washington, DC 20052, USA.
BMC Mol Biol. 2007 Aug 15;8:68. doi: 10.1186/1471-2199-8-68.
Recent analysis of immune-related genes within the sea urchin genome revealed a number of large gene families with vertebrate homologues, such as the Toll-like and NOD/NALP-like receptor families and C-type lectins in addition to a rudimentary complement system. Therefore, the immune response of the purple sea urchin appears to be more complex than previously believed. Another component of the sea urchin immune response is an unusual family of mRNAs, known as 185/333, which is strongly upregulated in response to pathogen challenge. The work presented here indicates that this family of transcripts is derived from an unexpectedly diverse gene family.
The 185/333 genes are small (< 2 kb) with only two exons. Their extraordinary diversity was exemplified by 121 unique sequences identified from 171 cloned genes. Sequences from the second exons were aligned optimally by introducing large gaps, which defined blocks of sequence known as elements. Genes were defined by the presence or absence of elements. Phylogenetic analysis defined five intron types which, when combined with the exon element patterns resulted in 31 gene patterns, 14 of which were not described previously. Sequence diversity was present in all elements, and was higher in the intron than the exons. Repeats within the sequence facilitated multiple alignments, of which two were analyzed in detail. Although the two alignments differed in length, number of elements, and number of patterns, both were about equally accurate at describing the 185/333 sequences. The genes were closely linked and flanked by short repeats. The repeats within and between the genes may promote their diversification through gene conversion, recombination, and meiotic mispairing.
The diversity of the 185/333 gene family represents an intriguing addition to what is known about the S. purpuratus immune response, and provides further evidence that invertebrate immune systems are neither simple nor static.
最近对海胆基因组中免疫相关基因的分析揭示了许多与脊椎动物同源的大基因家族，如Toll样和NOD/NALP样受体家族以及C型凝集素，此外还有一个基本的补体系统。因此，紫海胆的免疫反应似乎比以前认为的更为复杂。海胆免疫反应的另一个组成部分是一类不寻常的mRNA，称为185/333，其在病原体攻击时会强烈上调。本文展示的研究表明，这类转录本源自一个出人意料的多样化基因家族。
185/333基因很小（<2 kb），只有两个外显子。从171个克隆基因中鉴定出的121个独特序列体现了它们的非凡多样性。通过引入大的缺口，第二外显子的序列得以最佳比对，这些缺口定义了被称为元件的序列块。基因由元件的存在与否来定义。系统发育分析确定了五种内含子类型，当与外显子元件模式相结合时，产生了31种基因模式，其中14种是以前未描述过的。所有元件中都存在序列多样性，且内含子中的多样性高于外显子。序列中的重复促进了多重比对，其中两个被详细分析。尽管这两个比对在长度、元件数量和模式数量上有所不同，但在描述185/333序列方面的准确性大致相同。这些基因紧密相连，两侧是短重复序列。基因内部和之间的重复可能通过基因转换、重组和减数分裂错配促进它们的多样化。
185/333基因家族的多样性为紫海胆免疫反应的已知内容增添了有趣的内容，并进一步证明无脊椎动物免疫系统既不简单也非静止不变。
