Atypical catabolite repression of Geobacillus kaustophilus iol operons in the presence of ribose.

Geobacillus kaustophilus HTA426 is a thermophilic Gram-positive bacterium that encodes the myo-inositol-inducible iol operons, the products of which form the myo-inositol catabolic pathway. Strain PS8 is a mutant of HTA426 that is defective in myo-inositol catabolism due to constitutive repression of the iol operons. Several spontaneous suppressor mutants of PS8, which restored growth on myo-inositol, were obtained. These suppressors had mutations that may affect the translation of crh. Inclusion of a plasmid-based copy of crh into the suppressor mutants concomitantly restored the repression of the iol operons. PS8 and its suppressor mutants shared a mutated allele of hprK(G268R) that may encode a defective HPr kinase/phosphorylase, which could lead to the accumulation of the phosphorylated form of Crh. Inclusion of a plasmid-based copy of the wild-type hprK gene into PS8 restored the induction of the iol operons. Conversely, inclusion of a plasmid-based copy of hprK(G268R) into HTA426 significantly repressed the iol operons. In HTA426, ribose, rather than glucose, repressed the iol operons. During the growth on ribose, the expression of ptsH, which encodes HPr, was kept at a low level, while that of crh was elevated. Deletion of hprK, ccpA (encoding global regulator CcpA) or the rbs operon (encoding the components for ribose catabolism) abolishes the ribose-induced repression of iol operons. These results suggest an atypical catabolite repression of the iol operons of G. kaustophilus HTA426, where HPrK phosphorylates Crh in the presence of ribose, and the phosphorylated form of Crh cooperates with CcpA to repress the transcription of the iol operons.