A cDNA encoding adenylate isopentenyltransferase (AIPT) was cloned from young leaves of mulberry (Morus alba) by a homology-based RT-PCR. A recombinant enzyme expressed in Escherichia coli catalyzed pretty] transfer from DMAPP to the N-6 amino group of ADP and ATP, respectively, while AMP was a poor substrate of the enzyme. Interestingly, M. alba AIPT also accepted dADP, dATP, CDP, and GDP as the pretty] acceptors, and IPP, HMBPP, and GPP as the prenyl donors, to produce a series of cytokinin analogs. In particular, it was remarkable that the enzyme accepted HMBPP to produce truns-zeatin riboside phosphates, which suggested that trans-zeatin may be also produced from adenosine phosphates and HMBPP. Finally, alanine-scanning mutagenesis of conserved D49, Y54, F93, F120, Y153, F157, W159, Y170, Y217, and Q255, resulted in significant loss of enzyme activity except Y170A, confirming the functional and structural importance of the residues. (c) 2007 Elsevier Inc. All rights reserved.