Moore Robert B, Ferguson Katherine M, Loh William K W, Hoegh-Guldberg Ove, Carter Dee A
School of Molecular and Microbial Biosciences, University of Sydney, NSW 2006, Australia.
Int J Syst Evol Microbiol. 2003 Nov;53(Pt 6):1725-34. doi: 10.1099/ijs.0.02594-0.
The chloroplast genes of dinoflagellates are distributed among small, circular dsDNA molecules termed minicircles. In this paper, we describe the structure of the non-coding region of the psbA minicircle from Symbiodinium: DNA sequence was obtained from five Symbiodinium strains obtained from four different coral host species (Goniopora tenuidens, Heliofungia actiniformis, Leptastrea purpurea and Pocillopora damicornis), which had previously been determined to be closely related using LSU rDNA region D1/D2 sequence analysis. Eight distinct sequence blocks, consisting of four conserved cores interspersed with two metastable regions and flanked by two variable regions, occurred at similar positions in all strains. Inverted repeats (IRs) occurred in tandem or "twin" formation within two of the four cores. The metastable regions also consisted of twin IRs and had modular behaviour, being either fully present or completely absent in the different strains. These twin IRs are similar in sequence to double-hairpin elements (DHEs) found in the mitochondrial genomes of some fungi, and may be mobile elements or may serve a functional role in recombination or replication. Within the central unit (consisting of the cores plus the metastable regions), all IRs contained perfect sequence inverses, implying they are highly evolved. IRs were also present outside the central unit but these were imperfect and possessed by individual strains only. A central adenine-rich sequence most closely resembled one in the centre of the non-coding part of Amphidinium operculatum minicircles, and is a potential origin of replication. Sequence polymorphism was extremely high in the variable regions, suggesting that these regions may be useful for distinguishing strains that cannot be differentiated using molecular markers currently available for Symbiodinium.
甲藻的叶绿体基因分布在称为微环的小型环状双链DNA分子中。在本文中，我们描述了共生藻中psbA微环非编码区的结构：DNA序列取自从四种不同珊瑚宿主物种（细指蜂巢珊瑚、辐射太阳珊瑚、紫星珊瑚和鹿角杯形珊瑚）获得的五个共生藻菌株，这些菌株先前通过 LSU rDNA 区域 D1/D2 序列分析被确定为密切相关。在所有菌株的相似位置出现了八个不同的序列块，由四个保守核心组成，中间穿插着两个亚稳定区域，并由两个可变区域侧翼。反向重复序列（IRs）以串联或“双生”形式出现在四个核心中的两个内。亚稳定区域也由双生IRs组成，具有模块化行为，在不同菌株中要么完全存在，要么完全不存在。这些双生IRs在序列上与一些真菌线粒体基因组中发现的双发夹元件（DHEs）相似，可能是可移动元件，或者可能在重组或复制中起功能作用。在中央单元（由核心加上亚稳定区域组成）内，所有IRs都包含完美的序列反向，这意味着它们高度进化。IRs也存在于中央单元之外，但这些是不完美的，并且仅为个别菌株所具有。一个富含腺嘌呤的中央序列与微小亚历山大藻微环非编码部分中心的一个序列最为相似，是一个潜在的复制起点。可变区域的序列多态性极高，表明这些区域可能有助于区分使用目前可用于共生藻的分子标记无法区分的菌株。
