Spiroplasma melliferum, a mollicute bacterium, utilizes arginine as an energy source through the arginine dihydrolase pathway. This study characterizes arginine uptake mechanisms in this organism. L-arginine uptake by intact cells was saturable with a Km of 40 μM and was inhibited by L-ornithine and partially by lysine, but not by protonophores or ionophores. Treatment with protease or sulfhydryl-blocking reagents markedly inhibited uptake. Using hybrid membrane vesicles prepared by fusing isolated spiroplasma membranes with asolectin-cholesterol vesicles, researchers demonstrated rapid arginine-ornithine exchange that occurred bidirectionally and was independent of ATP. The exchange was electroneutral, as both arginine and ornithine are positively charged. These findings indicate that S. melliferum employs a precursor-product antiport system where the concentration gradients generated during arginine metabolism drive uptake without requiring metabolic energy. This mechanism is energetically favorable for an organism where arginine catabolism yields only one ATP per molecule metabolized. The arginine-ornithine exchanger appears to be constitutively expressed rather than induced by arginine availability.

Key findings

• L-arginine uptake in S. melliferum is a saturable, carrier-mediated process with Km of 40 μM that is inhibited by L-ornithine and requires intact protein and sulfhydryl groups
• An ATP-independent arginine-ornithine exchange system operates bidirectionally in sealed membrane vesicles, driven by concentration gradients of substrate and product
• The exchange system is electroneutral and independent of proton motive force, making it energetically favorable for arginine utilization
• The arginine-ornithine antiporter is constitutively expressed in S. melliferum, unlike the inducible system in some lactic acid bacteria