Transcript splicing characterization of tomato Ca2+-ATPase (LCA1 gene) mRNA indicates that two main transcripts are differentiated in the 3' terminal region. One of them contains a sequence of about 90 bp that could correspond to an untranslated intron that displays sequence homology to calmodulin-binding regions. Calmodulin-binding experiments demonstrate that only one of the two isoforms encoded by LCA1 binds to calmodulin. Since the M-w calculated for this peptide is 3.7 kDa, it is suggested that the presence of this intron is accounted for by the difference in the sizes of the two 116- and 120-kDa isoforms, and it determines calmodulin regulation. This represents a new strategy for a single gene to produce two isoforms that are localized differently (TN and PM), and which are either dependent on or independent of the calmodulin, which in turn is either regulated by the presence or by the absence of a 90 bp untranslated intron. (C) 2003 Elsevier Inc. All rights reserved.