CYANOBACTERIAL ACCLIMATION TO CHANGING ENVIRONMENTAL CONDITIONS Roles for group 2 sigma factors in Synechocystis sp. PCC 6803
Year: 2008
Type: dissertation
Abstract: In cyanobacteria gene expression is mainly controlled at the level of transcription initiation.The RNA polymerase holoenzyme (RNAP) is composed of a catalytic core and a sigma factor ( ) that recognizes promoter elements.Bacterial genomes encode several different factors and the exchange of one factor to another is thought to be a major determinant in changing the gene expression pattern.The cyanobacterium Synechocystis sp.PCC 6803 encodes nine factors.SigA is the group 1 factor, SigB, SigC, SigD and SigE are group 2 factors and SigF, SigG, SigH and SigI belong to group 3.Structural modelling of the RNAP revealed that the three-dimensional structures of the group 2 factors are very similar.Single, double and triple inactivation strains were constructed to study the roles of the group 2 factors.Similar growth and photosynthesis in all inactivation strains under standard conditions (BG-11 medium pH 7.5, 32 o C, continuous light 40 mol photons m -2 s -1 , air level CO 2 ) indicated that the group 2 factors are nonessential for growth under standard conditions.G r o u p 2 f a c t o r s w e r e f o u n d t o b e i m p o r t a n t f o r a c c l i m a t i o n t o e n v i r o n m e n t a l s t r e s s conditions.SigB and SigC have key roles in acclimation to heat stress.The SigB factor was required for survival in short extreme heat stress and for acquired thermotolerance.SigB was a positive regulator of the hspA gene, which encodes a heat shock protein.SigC in contrast was crucial both in long-and short-term heat stress and for acquired thermotolerance.High temperatures diminish the availability of inorganic carbon.DNA microarray analysis and physiological measurements suggest a role for the SigC factor in the regulation of carbon metabolism during heat acclimation.Optimal acclimation to salt and hyperosmotic stress involved many group 2 factors.SigB was crucial for growth in both salt and hyperosmotic stress while SigC played a minor role.The SigE factor had a role only in salt-induced osmotic stress.Acclimation to different light conditions was regulated by the SigB and SigD factors together.Strains lacking the SigD factor were sensitive to light on agar plates.When growth light intensity was doubled, all strains with simultaneously inactivated sigB and sigD genes were not able to take full advantage of a greater light availability.In addition, the sigBD strain was more vulnerable to high light stress than the control or single inactivation strains due to a deficient photosystem II repair cycle.The SigB and SigD factors have partial functional redundancy probably because they have highly similar structures. Don't lose your panic PhotoinhibitionAlthough light is an essential source of energy for photosynthesis, it also has destructive power.High-energy UV light is especially harmful to photosynthesis (He and Häder 2002, Hollósy 2002, Murata et al. 2007).In the light the PSII reaction centre is inactivated and the D1 protein is damaged.Inactivated PSII centres constantly undergo a repair cycle, which requires the removal and de novo synthesis of the D1 protein (Baena-González and Aro, 2002).When the rate of damage to the PSII reaction centre exceeds the rate of repair, oxygen evolution capacity decreases.This process is called photoinhibition (for review see Nishiyama et al. 2005, Murata
Source: UTUPub (University of Turku)
Author: Maija Pollari
Language: English
Subfield: Genetics
Domain: Life Sciences
Open Access status: green
Funders: Suomen Kulttuurirahasto, Turun Yliopisto
OpenAlex ID: W2279701008