Two extracellular endo-beta-1,4-mannanases, MAN I (major form) and MAN II (minor form), were purified to electrophoretic homogeneity from a locust bean gum-spent culture fluid of Aspergillus fumigatus IMI 385708 (formerly Thermomyces lanuginosus IMI 158749). Molecular weights of MAN I and MAN II estimated by SDS-PAGE were 60 and 63 kDa, respectively. IEF afforded several glycoprotein bands with pI values in the range of 4.9-5.2 for MAN I and 4.75-4.9 for MAN II, each exhibiting enzyme activity. MAN I as well as MAN II showed highest activity at pH 4.5 and 60 degrees C and were stable in the pH range 4.5-8.5 and up to 55 degrees C. In accordance with the ability of the enzymes to catalyze transglycosylation reactions, 1H NMR spectroscopy of reaction products generated from mannopentaitol confirmed the retaining character of both enzymes. Both MAN I and MAN II exhibited essentially identical kinetic parameters for polysaccharides and a similar hydrolysis pattern of various oligomeric and polymeric substrates. Both beta-mannanases contained identical internal amino acid sequence corresponding to glycoside hydrolase family 5 and also a cellulose-binding module. These data suggested that both MAN I and MAN II are products of the same gene differing in posttranslational modification. Indeed, the corresponding gene was identified within the recently sequenced Aspergillus fumigatus genome (http://sanger.ac.uk/Projects/A_fumigatus/).