Molecular cloning and sequencing of the gene encoding an exopolygalacturonase of a Bacillus isolate and properties of its recombinant enzyme.

An exopolygalacturonase (exo-PGase; EC 3.2.1.82) was found in the culture broth of a Bacillus isolate. The gene encoding the exo-PGase, pehK, was cloned by polymerase chain reaction using mixed primers designed from N-terminal and internal amino acid (aa) sequences of the enzyme (PehK). The determined nucleotide (nt) sequence of pehK revealed a 2940 bp open reading frame (980 aa) that encoded a putative signal sequence (27 aa) and a mature protein (953 aa; 103810 Da). The recombinant enzyme was purified to homogeneity from a culture broth of Bacillus subtilis harboring a pehK-containing plasmid. It had a molecular mass of 105 kDa and a pI value of 5.0. The maximum activity was observed at pH 8 and 55 degrees C in Tris-HCl buffer. The degradation products from polygalacturonic or oligogalacturonic acids were digalacturonic acid, like the exo-PGases, PehX of Erwinia chrysanthemi and PehB of Ralstonia solanacearum. The deduced aa sequence of PehK exhibited moderate homology to those of PehX and PehB with approx. 30% identity for both. High homology was observed in a suitably aligned internal region of the three enzymes (65% identity), and some of the conserved aa residues appeared to form the catalytic core of the enzymes.