Amino Acid Transport by Prosthecae of Asticcacaulis biprosthecum: Evidence for a Broad-range Transport System

This study investigates amino acid transport systems in prosthecae (cellular appendages) of the bacterium Asticcacaulis biprosthecum. Researchers isolated and purified prosthecae and demonstrated they contain active transport systems capable of accumulating all 20 amino acids tested against concentration gradients, using ascorbate-reduced phenazine methosulfate as an energy source. Through inhibition and exchange experiments, they identified three distinct transport systems: a general (G) system transporting at least 18 amino acids; a proline (P) system transporting seven amino acids including proline; and an acidic amino acid (A) system specific to glutamate and aspartate. Transport was energy-dependent, requiring respiration and oxygen, and was not blocked by ATP synthesis inhibition. Analysis of amino acid analogue-resistant mutants revealed differential transport defects, confirming the presence of multiple transport systems. The prosthecae appear to function in nutrient acquisition, potentially conferring a competitive advantage in nutrient-poor environments by increasing surface area for transport.

Key findings

• Isolated prosthecae from A. biprosthecum contain active amino acid transport systems capable of accumulating all 20 amino acids tested at concentrations 500-2000 times higher than external levels
• Three distinct amino acid transport systems were identified: a general system (G) transporting 18+ amino acids, a proline-specific system (P) transporting 7 amino acids, and an acidic amino acid system (A) specific to glutamate and aspartate
• Amino acid transport requires respiration and oxygen but is independent of ATP synthesis, indicating a proton-motive force-dependent mechanism
• Analogue-resistant mutants showed differential transport defects, with some mutants losing transport of 11 amino acids while retaining transport of nine amino acids, confirming multiple transport systems