Noll Kenneth M, Lapierre Pascal, Gogarten J Peter, Nanavati Dhaval M
Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT 06269-3125, USA.
BMC Evol Biol. 2008 Jan 15;8:7. doi: 10.1186/1471-2148-8-7.
The mal genes that encode maltose transporters have undergone extensive lateral transfer among ancestors of the archaea Thermococcus litoralis and Pyrococcus furiosus. Bacterial hyperthermophiles of the order Thermotogales live among these archaea and so may have shared in these transfers. The genome sequence of Thermotoga maritima bears evidence of extensive acquisition of archaeal genes, so its ancestors clearly had the capacity to do so. We examined deep phylogenetic relationships among the mal genes of these hyperthermophiles and their close relatives to look for evidence of shared ancestry.
We demonstrate that the two maltose ATP binding cassette (ABC) transporter operons now found in Tc. litoralis and P. furiosus (termed mal and mdx genes, respectively) are not closely related to one another. The Tc. litoralis and P. furiosus mal genes are most closely related to bacterial mal genes while their respective mdx genes are archaeal. The genes of the two mal operons in Tt. maritima are not related to genes in either of these archaeal operons. They are highly similar to one another and belong to a phylogenetic lineage that includes mal genes from the enteric bacteria. A unique domain of the enteric MalF membrane spanning proteins found also in these Thermotogales MalF homologs supports their relatively close relationship with these enteric proteins. Analyses of genome sequence data from other Thermotogales species, Fervidobacterium nodosum, Thermosipho melanesiensis, Thermotoga petrophila, Thermotoga lettingae, and Thermotoga neapolitana, revealed a third apparent mal operon, absent from the published genome sequence of Tt. maritima strain MSB8. This third operon, mal3, is more closely related to the Thermococcales' bacteria-derived mal genes than are mal1 and mal2. F. nodosum, Ts. melanesiensis, and Tt. lettingae have only one of the mal1-mal2 paralogs. The mal2 operon from an unknown species of Thermotoga appears to have been horizontally acquired by a Thermotoga species that had only mal1.
These data demonstrate that the Tc. litoralis and P. furiosus mdx maltodextrin transporter operons arose in the Archaea while their mal maltose transporter operons arose in a bacterial lineage, but not the same lineage as the two maltose transporter operons found in the published Tt. maritima genome sequence. These Tt. maritima maltose transporters are phylogenetically and structurally similar to those found in enteric bacteria and the mal2 operon was horizontally transferred within the Thermotoga lineage. Other Thermotogales species have a third mal operon that is more closely related to the bacterial Thermococcales mal operons, but the data do not support a recent horizontal sharing of that operon between these groups.
编码麦芽糖转运蛋白的mal基因在嗜热栖热球菌（Thermococcus litoralis）和激烈火球菌（Pyrococcus furiosus）的古菌祖先之间经历了广泛的横向转移。嗜热栖热菌目（Thermotogales）的细菌嗜热菌生活在这些古菌之中，因此可能参与了这些转移。海栖热袍菌（Thermotoga maritima）的基因组序列证明其广泛获取了古菌基因，所以其祖先显然具备这样的能力。我们研究了这些嗜热菌及其近亲的mal基因之间的深层系统发育关系，以寻找共同祖先的证据。
我们证明，目前在嗜热栖热球菌和激烈火球菌中发现的两个麦芽糖ATP结合盒（ABC）转运操纵子（分别称为mal和mdx基因）彼此之间没有密切关系。嗜热栖热球菌和激烈火球菌的mal基因与细菌mal基因关系最为密切，而它们各自的mdx基因则是古菌来源的。海栖热袍菌中两个mal操纵子的基因与这两个古菌操纵子中的任何一个基因都没有关系。它们彼此高度相似，属于一个系统发育谱系，其中包括来自肠道细菌的mal基因。在这些嗜热栖热菌属MalF同源物中也发现的肠道MalF跨膜蛋白的一个独特结构域支持了它们与这些肠道蛋白的相对密切关系。对其他嗜热栖热菌属物种、结节炽热杆菌（Fervidobacterium nodosum）、美拉尼西亚嗜热吸管菌（Thermosipho melanesiensis）、嗜岩石热袍菌（Thermotoga petrophila）、莱氏热袍菌（Thermotoga lettingae）和那不勒斯热袍菌（Thermotoga neapolitana）的基因组序列数据的分析揭示了第三个明显的mal操纵子，在海栖热袍菌菌株MSB8的已发表基因组序列中不存在。这个第三个操纵子mal3与嗜热球菌目细菌来源的mal基因比mal1和mal2更密切相关。结节炽热杆菌、美拉尼西亚嗜热吸管菌和莱氏热袍菌只有mal1 - mal2旁系同源物中的一个。来自未知热袍菌属物种的mal2操纵子似乎是由只有mal1的热袍菌属物种通过水平转移获得的。
这些数据表明，嗜热栖热球菌和激烈火球菌的mdx麦芽糖糊精转运操纵子起源于古菌，而它们的mal麦芽糖转运操纵子起源于细菌谱系，但与已发表序列的海栖热袍菌基因组中的两个麦芽糖转运操纵子不是同一谱系。这些海栖热袍菌麦芽糖转运蛋白在系统发育和结构上与肠道细菌中的相似，并且mal2操纵子在热袍菌谱系内进行了水平转移。其他嗜热栖热菌属物种有第三个mal操纵子，它与细菌嗜热球菌目mal操纵子关系更密切，但数据不支持该操纵子近期在这些类群之间的水平共享。
