de Koning Audrey P, Keeling Patrick J
Department of Botany, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada.
BMC Biol. 2006 Apr 21;4:12. doi: 10.1186/1741-7007-4-12.
Loss of photosynthesis has occurred independently in several plant and algal lineages, and represents a major metabolic shift with potential consequences for the content and structure of plastid genomes. To investigate such changes, we sequenced the complete plastid genome of the parasitic, non-photosynthetic green alga, Helicosporidium.
The Helicosporidium plastid genome is among the smallest known (37.5 kb), and like other plastids from non-photosynthetic organisms it lacks all genes for proteins that function in photosynthesis. Its reduced size results from more than just loss of genes, however; it has little non-coding DNA, with only one intron and tiny intergenic spaces, and no inverted repeat (no duplicated genes at all). It encodes precisely the minimal complement of tRNAs needed to translate the universal genetic code, and has eliminated all redundant isoacceptors. The Helicosporidium plastid genome is also highly structured, with each half of the circular genome containing nearly all genes on one strand. Helicosporidium is known to be related to trebouxiophyte green algae, but the genome is structured and compacted in a manner more reminiscent of the non-photosynthetic plastids of apicomplexan parasites.
Helicosporidium contributes significantly to our understanding of the evolution of plastid DNA because it illustrates the highly ordered reduction that occurred following the loss of a major metabolic function. The convergence of plastid genome structure in Helicosporidium and the Apicomplexa raises the interesting possibility that there are common forces that shape plastid genomes, subsequent to the loss of photosynthesis in an organism.
光合作用的丧失在多个植物和藻类谱系中独立发生，代表了一种主要的代谢转变，可能对质体基因组的内容和结构产生影响。为了研究此类变化，我们对寄生性非光合绿藻螺旋孢子虫的完整质体基因组进行了测序。
螺旋孢子虫的质体基因组是已知最小的质体基因组之一（37.5 kb），与其他非光合生物的质体一样，它缺乏所有参与光合作用的蛋白质基因。然而，其基因组大小的减小不仅仅是由于基因的丢失；它几乎没有非编码DNA，只有一个内含子和极小的基因间隔区，并且没有反向重复序列（根本没有重复基因）。它精确编码翻译通用遗传密码所需的最小tRNA互补序列，并消除了所有冗余的同功受体。螺旋孢子虫的质体基因组结构也高度有序，环形基因组的每一半在一条链上几乎包含所有基因。已知螺旋孢子虫与绿藻纲绿藻有关，但该基因组的结构和压缩方式更类似于顶复门寄生虫的非光合质体。
螺旋孢子虫极大地有助于我们理解质体DNA的进化，因为它说明了在主要代谢功能丧失后发生的高度有序的基因减少。螺旋孢子虫和顶复门的质体基因组结构趋同，这引发了一种有趣的可能性，即在生物体光合作用丧失后，存在塑造质体基因组的共同力量。
