Abstract
Several aspects of alginate and PHB synthesis in Azotobacter vinelandii at a molecular level have been elucidated in articles published during the last ten years. It is now clear that alginate and PHB synthesis are under a very complex genetic control. Genetic modification of A. vinelandii has produced a number of very interesting mutants which have particular traits for alginate production. One of these mutants has been shown to produce the alginate with the highest mean molecular mass so far reported. Recent work has also shed light on the factors determining molecular mass distribution; the most important of these being identified as; dissolved oxygen tension and specific growth rate. The use of specific mutants has been very useful for the correct analysis and interpretation of the factors affecting polymerization. Recent scale-up/down work on alginate production has shown that oxygen limitation is crucial for producing alginate of high molecular mass, a condition which is optimized in shake flasks and which can now be reproduced in stirred fermenters. It is clear that the phenotypes of mutants grown on plates are not necessarily reproducible when the strains are tested in lab or bench scale fermenters. In the case of PHB, A. vinelandii has shown itself able to produce relatively large amounts of this polymer of high molecular weight on cheap substrates, even allowing for simple extraction processes. The development of fermentation strategies has also shown promising results in terms of improving productivity. The understanding of the regulatory mechanisms involved in the control of PHB synthesis, and of its metabolic relationships, has increased considerably, making way for new potential strategies for the further improvement of PHB production. Overall, the use of a multidisciplinary approach, integrating molecular and bioengineering aspects is a necessity for optimizing alginate and PHB production in A. vinelandii.
Highlights
Alginates form an important family of biopolymers of both technological and scientific interest
This observation is supported by the fact that stress sigma factors AlgU and RpoS are key regulators for alginate synthesis and that a signal derived from cell wall damage triggers alginate production [30,33,42]
Other mutations which have a stimulatory effect on alginate yield, such as the one blocking PHB synthesis [46], have a negative effect on the growth capacity of A. vinelandii
Summary
Alginates form an important family of biopolymers of both technological and scientific interest. The following regulation model was proposed: in exponentially growing cells, the balanced growth conditions inhibit the β-ketothiolase activity which is present, and there is a low transcription of phbBAC caused by both the lack of RpoS, which affects transcription from one of the promoters of the PHB biosynthetic operon, and by the low concentraction of PhbR, whose transcription is partially dependent on this sigma factor [36]. The two-component global regulatory system formed by the sensor kinase GacS [32] and its corresponding response regulator GacA [33] is involved in the control of PHB production in A. vinelandii (Figure 2). 3.1 Alginate production by A. vinelandii mutants with increased algD transcription As previously discussed, an extensive work has been carried out, in order to unveil the regulatory network which controls alginate synthesis in A. vinelandii [24,2833,42,43].
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