Purification and Properties of a Thermostable Pullulanase from a Newly Isolated Thermophilic Anaerobic Bacterium, Fervidobacterium pennavorans Ven5

Abstract

Extremely thermophilic anaerobic fermentative bacteria growing at temperatures between 50 and 80(deg)C (optimum, 65 to 70(deg)C) were isolated from mud samples collected at Abano Terme spa (Italy). The cells were gram-negative motile rods, about 1.8 (mu)m in length and 0.6 (mu)m in width, occurring singly and in pairs. Cells commonly formed spheroids at one end similar to Fervidobacterium islandicum and Fervidobacterium nodosum. The new isolate differs from F. nodosum by the 7% higher G+C content of its DNA (40.6 mol%) but is similar to Fervidobacterium pennavorans and F. islandicum in its G+C content and phenotypic properties. The phylogenetic dendrogram indicates that strain Ven5 belongs to the order Thermotogales and shows the highest 16S ribosomal DNA sequence similarity to F. pennavorans, F. islandicum, and F. nodosum, with similarities of 99.0, 98.6, and 96.0%, respectively. During growth on starch the strain produced a thermostable pullulanase of type I which preferentially hydrolyzed (alpha)-1,6 glucosidic linkages. The enzyme was purified 65-fold by anion-exchange, gel permeation, and hydrophobic chromatography. The native pullulanase has a molecular mass of 240,000 Da and is composed of three subunits, each with a molecular mass of 77,600 Da as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Optimal conditions for the activity and stability of the purified pullulanase were pH 6.0 and 85(deg)C. At pH 6.0, the half-life of the enzyme was over 2 h at 80(deg)C and 5 min at 90(deg)C. This is the first report on the presence of pullulanase type I in an anaerobic bacterium.