Group of Prof. Dr. M. Thomm
coworkers: Melanie Surma, PD Dr. Winfried Hausner


TrmB and TrmBL1, two transcriptional regulators in the heart of the cross regulatory

 

Pyrococcus furiosus is a hyperthermophilic archaeon that is able to utilize carbohydrates like maltose, trehalose, and maltodextrins as carbon and energy source (Fiala & Stetter, 1986). The uptake of these substrates is mediated by binding protein dependent ABC-transporters which consist of three different kinds of proteins: two permeases, that form the translocation pore, two ATPase domains, that provide energy for the translocation by hydrolysis of ATP, and a binding protein with high affiniy to its substrate (Konig et al., 2002).
In Pyrococcus furiosus two substrate specific ABC-transporters have been found – one responsible for the uptake of maltose/trehalose, the other for the transport of maltodextrins (Figure 1).

Fig 1

The genes encoding the different transport proteins are clustered in an operon and their expression is regulated together. They are induced only in the presence of specific substrates, i.e. maltose and trehalose for the maltose/trehalose transporter, and maltodextrins for the maltodextrin transporter.

In both cases the expression of the transport proteins is controlled by the regulator TrmB (transcriptional factor of the mal operon; Lee et al. 2005). TrmB is a repressor of transcription. In the absence of appropriate substrates it binds to the promoter region of the respective gene complex and thus inhibits transcription. In the presence of an inducer, however, causes a conformational change in the repressor, it looses contact of the promoter and transcription can take place again.

TrmB has a unique characteristic: it recognizes different DNA-sequences on different promoters, takes up different conformations upon binding to them, and thus recognizes different inducers for different promoters. So TrmB shows a different sugar specificity that depends on the nucleotide sequence of the TrmB binding site (Figure 2). As a consequence one and the same sugar can act as an inducer or as an anti-inducer, depending on the promoter TrmB is bound to.

Fig. 2

Besides TrmB three other TrmB orthologues have been found in Pyrococcus furiosus, called TrmBL1, TrmBL2, and TrmBL3. These are also repressors of transcription and are involved in the regulation of glycolytic genes. In vitro transcription assays showed that TrmB also recognizes the target promoters of TrmBL1-3, and vice versa. This leads to the conclusion that the sugar metabolism in Pyrococcus furiosus is controlled by a cross regulatory network of homologue repressor proteins. Furthermore, recent experiments using gluconeogenesis genes indicate that TrmBL1 does not act as an repressor or these promoters, but as an activator.