The Arabidopsis AHL gene encodes a 3'(2'),5'-bisphosphate nucleotidase (BPNTase) involved in the reductive sulfate activation pathway. A bacterial expression vector containing AHL cDNA was randomly mutagenized with hydroxylamine and transformed into the E. coli cysteine auxotrophic mutant cysQ. Bacterial colonies that did not show evidence of complementation, i.e. those that exhibited slower growth on cysteine-free medium, were selected for further study. Sequencing of the AHL cDNA in one such clone revealed the conversion of cytosine 635 (C-635) to thymine, resulting in an Alanine (A(212)) to Valine substitution. This microbial complementation procedure is useful in BPNTase structure-activity studies for biotechnological applications.