Genetics of bacterial alginate: alginate genes distribution, organization and biosynthesis in bacteria

Abstract

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.

Keywords: Azotobacter; Pseudomonads; alginate genes; biosynthesis; regulation.

Fig. (1)

Model of the regulation of alginate biosynthesis and physical map (unshaded pentagons along with circle) of alginate structural/biosynthetic (10/34-min), regulatory (9/13-min) and genetic switching (68-min), genes at chromosome in Pseudomonas aeruginosa. Heads of pentagons indicate direction of transcription and shaded shapes in side the circular chromosome are the gene products. CSK(cognate sensor Kinase), RRP(response regulatory protein), CAP(catabolite activator protein), + and − (positive and negative effect).

Fig. (2)

Model for the biosynthesis and assembly of alginate in P. aeruginosa [147]. I.M. and O.M(inner and outer membrane respectively), G and M(guluronate and mannuronate respectively) and Ac(acetyl group).