Abstract
CelR, a protein that regulates transcription of cellulase genes in Thermomonospora fusca (Actinomycetaceae) was purified to homogeneity. A 6-kilobase NotI-SacI fragment of T. fusca DNA containing the celR gene was cloned into Esherichia coli and sequenced. The celR gene encodes a 340-residue polypeptide that is highly homologous to members of the GalR-LacI family of bacterial transcriptional regulators. CelR specifically binds to a 14-base pair inverted repeat, which has sequence similarity to the binding sites of other family members. This site is present in regions upstream of all six cellulase genes in T. fusca. The binding of CelR to the celE promoter is inhibited specifically by low concentrations of cellobiose (0.2-0.5 mM), the major end product of cellulases. The other sugars tested did not affect binding at equivalent or 50-fold higher concentrations. The results suggest that CelR may act as a repressor, and that the mechanism of induction involves a direct interaction of CelR with cellobiose.
Highlights
The thermophilic actinomycete, Thermomonospora fusca, a major degrader of cellulose in plant residues, is an extensively studied cellulolytic bacterium
Biosynthesis of cellulases in T. fusca is regulated by cellobiose induction and catabolite repression, with any readily metabolized sugar acting as a repressor [6]
The same palindrome was found in the regulatory regions of cellulase genes from different streptomycetes, in particular, the cenC gene from Streptomyces strain KSM9 [13], the celA genes from Streptomyces halstedii [14] and Streptomyces lividans [15], and in the cel1 gene from Streptomyces reticuli [16], where it serves as the operator for a repressor protein [17]
Summary
The thermophilic actinomycete, Thermomonospora fusca, a major degrader of cellulose in plant residues, is an extensively studied cellulolytic bacterium. This species produces six different extracellular cellulases, designated E1 through E6. All six cellulase genes in T. fusca contain the 14-bp inverted repeat sequence TGGGAGCGCTCCCA in their 5Јupstream regions. A regulatory protein that interacts with the upstream regions of cel genes was detected in T. fusca cultures grown on cellulose. The same sequence was found in a xylanase gene from Thermomonospora alba [18] The occurrence of this site in cellulase and xylanase genes from different species may indicate its special role in regulating carbohydrate catabolism in streptomycetes and other high GC Gram-positive bacteria
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