Arginine is an essential amino acid for the human pathogen Leishmania but not to its host. Thus, the mechanism by which this protozoan parasite regulates cellular homeostasis of arginine is critical for its survival and virulence. In a previous study, we cloned and functionally characterized a high affinity arginine-specific transporter, LdAAP3, from Leishmania donovani. In this investigation, we have characterized the relationship between arginine transport via LdAAP3 and amino acid availability. Starving promastigotes for amino acids decreased the cellular level of most amino acids including arginine but also increased the abundance of both LdAAP3 mRNA and protein and up-regulated arginine transport activity. Genetic obliteration of the polyamine biosynthesis pathway for which arginine is the sole precursor caused a significant decrease in the rate of arginine transport. Cumulatively, we established that LdAAP3 expression and activity changed whenever the cellular level of arginine changed. Our findings have led to the hypothesis that L. donovani promastigotes have a signaling pathway that senses cellular concentrations of arginine and subsequently activates a mechanism that regulates LdAAP3 expression and activity. Interestingly, this response of LdAAP3 to amino acid availability in L. donovani is identical to that of the mammalian cation amino acid transporter 1. Thus, we conjecture that Leishmania mimics the host response to amino acid availability to improve virulence.L-Arginine is a metabolically versatile cationic amino acid that provides a precursor function for the synthesis of a variety of bioactive molecules in all organisms. These molecules include nitric oxide, polyamines, phospho-and methylarginine, and proteins. Most of these molecules are essential for cell growth, and a few, like nitric oxide, are also used to protect hosts from pathogen invasion (1, 2). To enable simultaneous feeding of multiple and disparate biosynthetic pathways, cells must maintain ample cellular levels of arginine at all times. A variety of mechanisms for arginine homeostasis have been identified in mammals and fungi (3, 4). However, only very limited information on arginine homeostasis in protozoan parasites and its role in pathogenesis is available. Our laboratory aims to address these questions in the human pathogen Leishmania. Protozoan parasites of the genus Leishmania are the causative agents of a wide spectrum of human and veterinary diseases. Infections with Leishmania vary in their clinical manifestations depending upon the species ranging from self-healing skin lesions to visceral pathogenesis, which is invariably fatal if untreated (5). Leishmania species exhibit a digenetic life cycle that includes both extracellular promastigote and intracellular amastigote forms. The extracellular promastigotes develop in the alimentary tract of sand flies, whereas amastigotes reside within macrophage phagolysosomes (6, 7).Arginine is an essential amino acid for Leishmania (8), and therefore its metabolism and homeostasis dep...