Centre for Molecular Genetics, University of Karachi, Karachi-75270, Pakistan.
Curr Genomics. 2007 May;8(3):191-202. doi: 10.2174/138920207780833810.
Bacterial alginate genes are chromosomal and fairly widespread among rRNA homology group I Pseudomonads and Azotobacter. In both genera, the genetic pathway of alginate biosynthesis is mostly similar and the identified genes are identically organized into biosynthetic, regulatory and genetic switching clusters. In spite of these similarities,still there are transcriptional and functional variations between P. aeruginosa and A. vinelandii. In P. aeruginosa all biosynthetic genes except algC transcribe in polycistronic manner under the control of algD promoter while in A. vinelandii, these are organized into many transcriptional units. Of these, algA and algC are transcribed each from two different and algD from three different promoters. Unlike P. aeruginosa, the promoters of these transcriptional units except one of algC and algD are algT-independent. Both bacterial species carry homologous algG gene for Ca(2+)-independent epimerization. But besides algG, A. vinelandii also has algE1-7 genes which encode C-5-epimerases involved in the complex steps of Ca(2+)-dependent epimerization. A hierarchy of alginate genes expression under sigma(22)(algT) control exists in P. aeruginosa where algT is required for transcription of the response regulators algB and algR, which in turn are necessary for expression of algD and its downstream biosynthetic genes. Although algTmucABCD genes cluster play similar regulatory roles in both P. aeruginosa and A. vinelandii but unlike, transcription of A. vinelandii, algR is independent of sigma(22). These differences could be due to the fact that in A. vinelandii alginate plays a role as an integrated part in desiccation-resistant cyst which is not found in P. aeruginosa.
细菌海藻酸盐基因是染色体的，在 rRNA 同源组 I 假单胞菌和固氮菌中相当普遍。在这两个属中，海藻酸盐生物合成的遗传途径大多相似，已鉴定的基因相同地组织成生物合成、调控和遗传转换簇。尽管存在这些相似性，但在铜绿假单胞菌和 A. vinelandii 之间仍存在转录和功能差异。在铜绿假单胞菌中，除 algC 外，所有生物合成基因都在 algD 启动子的控制下以多顺反子方式转录，而在 A. vinelandii 中，这些基因被组织成许多转录单位。其中，algA 和 algC 分别从两个不同的启动子转录，而 algD 从三个不同的启动子转录。与铜绿假单胞菌不同，这些转录单位的启动子除 algC 和 algD 的一个外，均不依赖于 algT。这两种细菌都携带同源的 algG 基因，用于 Ca(2+)非依赖性差向异构化。但除了 algG，A. vinelandii 还具有 algE1-7 基因，这些基因编码参与 Ca(2+)依赖性差向异构化的复杂步骤的 C-5-差向异构酶。在铜绿假单胞菌中，alginate 基因的表达存在 sigma(22)(algT)控制的层次结构，其中 algT 是转录应答调节因子 algB 和 algR 的必需条件，而 algR 又必需表达 algD 及其下游生物合成基因。尽管 algTmucABCD 基因簇在铜绿假单胞菌和 A. vinelandii 中发挥相似的调节作用，但与 A. vinelandii 不同的是，algR 的转录不依赖于 sigma(22)。这些差异可能是由于在 A. vinelandii 中，海藻酸盐作为干燥抗性胞囊的一个组成部分发挥作用，而在铜绿假单胞菌中不存在这种胞囊。
