Isolation of novel strains of Agrobacterium radiobacter with altered capacities for lactose metabolism and succinoglucan production

Abstract

SUMMARY: Agrobacterium radiobacter NCIB 11883 was grown in continuous culture at low dilution rate under lactose limitation. Washed cells rapidly transported lactose [and its non-metabolizable analogue MTG (methyl β-d-thiogalactoside)] via a lactose-binding protein (LBP)-dependent uptake system, and subsequently hydrolysed the lactose using a highly active β-galactosidase composed of two identical subunits of M_r approximately 86000. Growth under these conditions for 0·45, indicating that lactose uptake is a major kinetic control point for polysaccharide production. Growth of strain AR50 under ammonia limitation for > 40 generations led to the selection of another novel strain (AR60) which exhibited a decreased q_p [0·16 g h⁻¹ (g cells)⁻¹]. Washed cells of strain AR60 exhibited significantly lower rates of lactose (MTG) uptake than strain AR50, an observation which was commensurate with the rate of polysaccharide production being predominantly controlled by the rate of lactose uptake, but β-galactosidase activity was substantially higher. Both the lactose uptake system and β-galactosidase were expressed constitutively in strain AR60, but catabolite repression of β-galactosidase was much weaker than in the wild-type organism or strain AR50.