The Nicotiana sylvestris nucleobase cation symporter 1 retains a dicot solute specificity profile

Abstract

Nucleobase Cation Symporter 1 proteins are present in bacteria, fungi and among plants. Microbes often contain numerous NCS1 proteins that have evolved specific solute specificity profiles (solute transport and binding). Conversely, most members of Viridiplantae have a unique NCS1 that shows a broad, yet species –specific, solute specificity profile. Properties of NCS1 from Nicotiana sylvestris Speg. & Comes (NsNCS1) have been determined as part of an ongoing evolution-function analysis through heterologous expression studies in ncs1-deficient Saccharomyces cerevisiae Meyen ex E.C. Hansen as well as functional assays and subcellular localization studies in planta. NsNCS1 transports the purine adenine with high affinity and moves guanine, uracil, cytosine and hypoxanthine. Xanthine, uric acid and 5-flourocytosine act as competitive inhibitors for NsNCS1. When expressed in Arabidopsis, NsNCS1 locates to the chloroplast and facilitates the transport of guanine and 8-azaguanine. The NsNCS1 solute specificity profile is very similar, yet distinct, from AtNCS1 (A. thaliana (L.) Heynh. NCS1) and indicates that despite evolutionary divergence in the Eudicots - Nicotiana, an Astrid, and Arabidopsis, a Rosid, − little functional difference evolved. The alignment of select plant NCS1 amino acid sequences coupled with a detailed knowledge of each solute specificity profile facilitates identifying residues that may contribute to subtle solute discrimination. Such an evolution-function analysis will complement recent molecular model based site-directed mutagenesis analysis of select microbial and plant NCS1.