Lavire Céline, Blaha Didier, Cournoyer Benoit
Research Group on Opportunistic Pathogens and the Environment, Ecologie Microbienne (Center for Microbial Ecology), UMR CNRS 5557-Université Claude Bernard-Lyon 1, USC INRA 1193, 69622 Villeurbanne Cedex, France.
Appl Environ Microbiol. 2004 Feb;70(2):991-8. doi: 10.1128/AEM.70.2.991-998.2004.
Functional adaptations of sigma70 transcriptional factors led to the emergence of several paralogous lineages, each one being specialized for gene transcription under particular growth conditions. Screening of a Frankia strain EaI-12 gene library by sigma70 DNA probing allowed the detection and characterization of a novel actinomycetal primary (housekeeping) sigma70 factor. Phylogenetic analysis positioned this factor in the RpoD cluster of proteobacterial and low-G+C-content gram-positive factors, a cluster previously free of any actinobacterial sequences. sigma70 DNA probing of Frankia total DNA blots and PCR screening detected one or two rpoD-like DNA regions per species. rpoD matched the conserved region in all of the species tested. The other region was found to contain sigA, an alternative primary factor. sigA appeared to be strictly distributed among Frankia species infecting plants by the root hair infection process. Both genes were transcribed by Frankia strain ACN14a grown in liquid cultures. The molecular phylogeny of the sigma70 family determined with Frankia sequences showed that the alternative actinomycetal factors and the essential ones belonged to the same radiation. At least seven distinct paralogous lineages were observed among this radiation, and gene transfers were detected in the HrdB actinomycetal lineage.
σ70转录因子的功能适应性导致了几个旁系同源谱系的出现，每个谱系在特定生长条件下专门负责基因转录。通过σ70 DNA探针筛选弗氏放线菌菌株EaI-12基因文库，得以检测和鉴定一种新型放线菌主要（管家）σ70因子。系统发育分析将该因子定位在变形菌和低G+C含量革兰氏阳性因子的RpoD簇中，该簇此前没有任何放线菌序列。对弗氏放线菌总DNA印迹进行σ70 DNA探针检测和PCR筛选，发现每个物种有一个或两个rpoD样DNA区域。rpoD与所有测试物种中的保守区域匹配。发现另一个区域含有sigA，一种替代主要因子。sigA似乎严格分布在通过根毛感染过程感染植物的弗氏放线菌物种中。这两个基因在液体培养物中生长的弗氏放线菌菌株ACN14a中都有转录。用弗氏放线菌序列确定的σ70家族的分子系统发育表明，替代放线菌因子和必需因子属于同一进化分支。在该进化分支中观察到至少七个不同的旁系同源谱系，并且在HrdB放线菌谱系中检测到基因转移。
