Transport of nucleosides in Bacillus subtilis: the effect of purine nucleosides on the cytidine-uptake.

The structural effects of chemical modifications upon the affinity of purine nucleosides to cytidine-transport system in Bacillus subtilis were investigated using a series of modified derivatives. The interaction involves protein molecule(s) which require the presence and proper orientation of the sugar residue and its hydroxylic functions. Moreover, a specific interaction with the heterocyclic ring system is involved in the process which results in a requirement for an aromatic pi -electron system and an absence of a polarizable function at position 6 of the purine heterocycle. The region in the protein responsible for the latter interaction is rather limited and, consequently, a proper nucleoside conformation is required.