SUBSTRATE SPECIFICITIES OF POLY(HYDROXYALKANOATE)-DEGRADING BACTERIA AND ACTIVE-SITE STUDIES ON THE EXTRACELLULAR POLY(3-HYDROXYOCTANOIC ACID) DEPOLYMERASE OF PSEUDOMONAS-FLUORESCENS GK13

The isolation of poly(3-hydroxyoctanoic acid)- and poly(6-hydroxyhexan oic acid)-degrading bacteria yielded 28 strains with abilities to degr ade various polymers. The most versatile strains hydrolyzed five diffe rent polyesters comprising short chain length and medium chain length poly(hydroxyalkanoates). The new isolates together with previously iso lated poly(hydroxyalkanoate)-degrading bacteria were classified into 1 1 groups with respect to their polymer-degrading specificities. All PH A depolymerases studied so far have been characterized by the lipase c onsensus sequence Gly-X-Ser-X-Gly in their amino acid sequence, which is a known sequence for serine hydrolases. When we replaced the centra l residue, Ser-172, in the corresponding sequence Gly-Ile-Ser-Ser-Gly of the extracellular poly(3-hydroxyoctanoic acid) depolymerase of Pseu domonas fluorescens GK13, with alanine the enzyme lost its activity co mpletely. This result of the mutational experiment indicates that the poly(3-hydroxyoctanoic acid) depolymerase belongs to the family of ser ine hydrolases.