Lima Wanessa C, Varani Alessandro M, Menck Carlos F M
Department of Microbiology, Institute of Biomedical Sciences, University of São Paulo, Brazil.
Mol Biol Evol. 2009 Feb;26(2):399-406. doi: 10.1093/molbev/msn261. Epub 2008 Nov 12.
The biosynthesis of quinolinate, the de novo precursor of nicotinamide adenine dinucleotide (NAD), may be performed by two distinct pathways, namely, the bacterial aspartate (aspartate-to-quinolinate) and the eukaryotic kynurenine (tryptophan-to-quinolinate). Even though the separation into eukaryotic and bacterial routes is long established, recent genomic surveys have challenged this view, because certain bacterial species also carry the genes for the kynurenine pathway. In this work, both quinolinate biosynthetic pathways were investigated in the Bacteria clade and with special attention to Xanthomonadales and Bacteroidetes, from an evolutionary viewpoint. Genomic screening has revealed that a small number of bacterial species possess some of the genes for the kynurenine pathway, which is complete in the genus Xanthomonas and in the order Flavobacteriales, where the aspartate pathway is absent. The opposite pattern (presence of the aspartate pathway and absence of the kynurenine pathway) in close relatives (Xylella ssp. and the order Bacteroidales, respectively) points to the idea of a recent acquisition of the kynurenine pathway through lateral gene transfer in these bacterial groups. In fact, sequence similarity comparison and phylogenetic reconstruction both suggest that at least part of the genes of the kynurenine pathway in Xanthomonas and Flavobacteriales is shared by eukaryotes. These results reinforce the idea of the role that lateral gene transfer plays in the configuration of bacterial genomes, thereby providing alternative metabolic pathways, even with the replacement of primary and essential cell functions, as exemplified by NAD biosynthesis.
喹啉酸是烟酰胺腺嘌呤二核苷酸（NAD）的从头合成前体，其生物合成可通过两条不同的途径进行，即细菌的天冬氨酸途径（从天冬氨酸到喹啉酸）和真核生物的犬尿氨酸途径（从色氨酸到喹啉酸）。尽管将其分为真核生物途径和细菌途径早已有之，但最近的基因组调查对这一观点提出了挑战，因为某些细菌物种也携带犬尿氨酸途径的基因。在这项工作中，从进化的角度对细菌类群中的喹啉酸生物合成途径进行了研究，并特别关注了黄单胞菌目和拟杆菌门。基因组筛选显示，少数细菌物种拥有犬尿氨酸途径的一些基因，在黄单胞菌属和黄杆菌目中该途径是完整的，而天冬氨酸途径则不存在。在近亲（分别为木质部菌属和拟杆菌目）中呈现出相反的模式（存在天冬氨酸途径而不存在犬尿氨酸途径），这表明这些细菌类群中最近通过横向基因转移获得了犬尿氨酸途径。事实上，序列相似性比较和系统发育重建均表明，黄单胞菌属和黄杆菌目中犬尿氨酸途径的至少部分基因是与真核生物共享的。这些结果强化了横向基因转移在细菌基因组构成中所起作用的观点，从而提供了替代代谢途径，甚至可以替代主要和基本的细胞功能，烟酰胺腺嘌呤二核苷酸生物合成就是一个例证。
