Possible roles for a non-modular, thermostable and proteinase-resistant cellulase from the mesophilic aerobic soil bacterium Cellvibrio mixtus |
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Authors: | C M G A Fontes J H Clarke G P Hazlewood T H Fernandes H J Gilbert L M A Ferreira |
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Institution: | (1) CIISA-Faculdade de Medicina Veterinária, Rua Gomes Freire, P-1199 Lisboa Codex, Portugal Tel.: +351 1 3522595 Fax: +351 1 3533088 e-mail: luisferr@fmv.utl.pt, PT;(2) Department of Cellular Physiology, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK, GB;(3) Department of Biological and Nutritional Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK, GB |
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Abstract: | The widespread presence of cellulose-binding domains in cellulases from aerobic bacteria and fungi suggests the existence
of a strong selective pressure for the retention of these non-catalytic modules. The complete nucleotide sequence of the cellulase
gene, celA, from the aerobic soil bacterium Cellvibrio mixtus, was determined. It revealed an open reading frame of 1089 bp that encoded a polypeptide, defined as cellulase A (CelA),
of M
r 41 548. CelA displayed features characteristic of an endo-β-1,4-glucanase, rapidly decreasing the viscosity of the substrate
while releasing only moderate amounts of reducing sugar. Deletion studies in celA revealed that removal of 78 nucleotides from the 5′ end or 75 from the 3′ end of the gene led to the complete loss of cellulase
activity of the encoded polypeptides. The deduced primary structure of CelA revealed an N-terminal signal peptide followed
by a region that exhibited significant identity with the catalytic domains of cellulases belonging to glycosyl hydrolase family
5. These data suggest that CelA is a single-domain endoglucanase with no distinct non-catalytic cellulose-binding domain.
Analysis of the biochemical properties of CelA revealed that the enzyme hydrolyses a range of soluble cellulosic substrates,
but was inactive against Avicel, xylan or any other hemicellulose. CelA was resistant to proteolytic inactivation by pancreatic
proteinases and surprisingly, in view of its mesophylic origin, was shown to be thermostable. The significance of these findings
in relation to the role of single-domain cellulases in plant cell wall hydrolysis by aerobic microorganisms is discussed.
Received: 26 May 1997 / Received revision: 4 July 1997 / Accepted: 4 July 1997 |
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