The Crl protein activates cryptic genes for curli formation and fibronectin binding in Escherichia coli HB101

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Summary: Curli are thin, coiled, temperature‐regulated fibres on fibronectin‐binding Escherichia coli. The subunit protein of curli was highly homologous at its amino terminus to SEF‐17, the subunit protein of thin, aggregative fimbriae of Salmonella enteritidis 27655 strain 3b, suggesting that these fibres form a novel class of surface organelles on enterobacteria. E. coli HB101 is non‐curliated and unable to bind soluble, iodinated fibronectin. The phenotypically cryptic curlin subunit gene, csgA, in HB101 is transcriptionally activated by expressing the cytoplasmic Crl on a multicopy plasmid. Transcriptional activation of csgA by Crl was observed after growth at 26°C but not at 37°C, even though crl transcription was not thermoregulated. A deletion of the 39 carboxy‐terminal residues abolished Crl activity, whereas a deletion of 10 residues at the C‐terminus did not, implying that a region between residue 93 and 122 in the 132‐amino‐acid‐residue large Crl protein is required for activating curli expression in E. coli HB101. crl is a normal housekeeeping gene in E. coli and it is suggested that its gene product may either be a DNA‐binding protein affecting chromatin structure as has been suggested for histone‐like protein H1 or interact with specific regulatory protein(s) controlling transcription of genes required for curli formation and fibronectin binding.

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