Summary Ss-LrpB is an Lrp-like transcriptional regulator fromSulfolobus solfataricus. Previously, in vitro binding of Ss-LrpB to the control region of its own gene has been extensively studied. However, nothing was known about the physiological role of this regulator yet. Here, using the knowledge of the DNA-binding sequence specificity of Ss-LrpB, several potential binding sites were predicted in silico in promoter regions of genes located adjacently to the Ss-lrpB gene. These genes include an operon encoding a pyruvate ferredoxin oxidoreductase (porDAB) and two genes encoding putative permeases. In vitro protein-DNA interaction studies allowed the identification of the Ss-LrpB binding sites in the cognate control regions. Intriguingly, the binding site organization in the por operator is identical to that in the Ss-lrpB control region. An Ss-lrpB gene disruption mutant was constructed and the gene expression of the above-mentioned targets in this mutant was analysed by qRT-PCR and compared with isogenic wild type. Our data demonstrate that in vivo Ss-LrpB acts as an activator at the promoters of the three predicted targets. Based on these results, it appears that not all regulators belonging to the archaeal Lrp family perform a function related to the amino acid metabolism, unlike the bacterial Lrp-like regulators.

Sa-Lrp is a member of the leucine-responsive regulatory protein (Lrp)-like family of transcriptional regulators in Sulfolobus acidocaldarius. Previously, we demonstrated the binding of Sa-Lrp to the control region of its own gene in vitro. However, the function and cofactor of Sa-Lrp remained an enigma. In this work, we demonstrate that glutamine is the cofactor of Sa-Lrp by inducing the formation of octamers and increasing the DNA-binding affinity and sequence specificity. In vitro protein-DNA interaction assays indicate that Sa-Lrp binds to promoter regions of genes with a variety of functions including ammonia assimilation, transcriptional control, and UV-induced pili synthesis. DNA binding occurs with a specific affinity for AT-rich binding sites, and the protein induces DNA bending and wrapping upon binding, indicating an architectural role of the regulator. Furthermore, by analyzing an Sa-lrp deletion mutant, we demonstrate that the protein affects transcription of some of the genes of which the promoter region is targeted and that it is an important determinant of the cellular aggregation phenotype. Taking all these results into account, we conclude that Sa-Lrp is a glutamine-responsive global transcriptional regulator with an additional architectural role.