School of Molecular & Cell Biology, University of the Witwatersrand, 2050 Wits, Johannesburg, South Africa.
Centre for Microbial Ecology and Genomics, University of Pretoria 0002, Pretoria, South Africa.
BMC Genomics. 2020 Jan 30;21(1):100. doi: 10.1186/s12864-020-6529-9.
The order Enterobacterales encompasses a broad range of metabolically and ecologically versatile bacterial taxa, most of which are motile by means of peritrichous flagella. Flagellar biosynthesis has been linked to a primary flagella locus, flag-1, encompassing ~ 50 genes. A discrete locus, flag-2, encoding a distinct flagellar system, has been observed in a limited number of enterobacterial taxa, but its function remains largely uncharacterized.
Comparative genomic analyses showed that orthologous flag-2 loci are present in 592/4028 taxa belonging to 5/8 and 31/76 families and genera, respectively, in the order Enterobacterales. Furthermore, the presence of only the outermost flag-2 genes in many taxa suggests that this locus was far more prevalent and has subsequently been lost through gene deletion events. The flag-2 loci range in size from ~ 3.4 to 81.1 kilobases and code for between five and 102 distinct proteins. The discrepancy in size and protein number can be attributed to the presence of cargo gene islands within the loci. Evolutionary analyses revealed a complex evolutionary history for the flag-2 loci, representing ancestral elements in some taxa, while showing evidence of recent horizontal acquisition in other enterobacteria.
The flag-2 flagellar system is a fairly common, but highly variable feature among members of the Enterobacterales. Given the energetic burden of flagellar biosynthesis and functioning, the prevalence of a second flagellar system suggests it plays important biological roles in the enterobacteria and we postulate on its potential role as locomotory organ or as secretion system.
肠杆菌目涵盖了广泛的代谢和生态多功能细菌分类群，其中大多数通过周生鞭毛进行运动。鞭毛生物合成与主要鞭毛基因座 flag-1 相关，该基因座包含约 50 个基因。在有限数量的肠杆菌目中观察到离散的 flag-2 基因座，该基因座编码独特的鞭毛系统，但它的功能在很大程度上尚未确定。
比较基因组分析表明，在肠杆菌目中，5/8 个科和 31/76 个属的 4028 个分类群中，有 592 个分类群具有同源 flag-2 基因座。此外，许多分类群中只有最外端的 flag-2 基因表明该基因座更为普遍，随后通过基因缺失事件丢失。flag-2 基因座的大小范围从约 3.4 到 81.1 千碱基，编码 5 到 102 个不同的蛋白质。大小和蛋白质数量的差异可归因于基因座内货物基因岛的存在。进化分析表明，flag-2 基因座的进化历史复杂，在一些分类群中代表祖先元素，而在其他肠杆菌中则显示出最近水平获得的证据。
flag-2 鞭毛系统是肠杆菌目中相当常见但高度可变的特征。鉴于鞭毛生物合成和功能的能量负担，第二个鞭毛系统的存在表明它在肠杆菌中具有重要的生物学作用，我们推测它可能作为运动器官或分泌系统发挥作用。
