CLONING AND CHARACTERIZATION OF AN APPLE (MALUS-DOMESTICA (L) BORKH) CDNA-ENCODING A CALMODULIN-BINDING PROTEIN DOMAIN SIMILAR TO THE CALMODULIN-BINDING REGION OF TYPE-II MAMMALIAN CA2+/CALMODULIN-DEPENDENT PROTEIN-KINASE

A cDNA expression library constructed in the lambda-gt11 vector with mRNA isolated from in vitro grown apple plantlets was screened with I-125-labeled calmodulin as a probe. One recombinant clone, lambda-CB1, produces a fusion protein that binds calmodulin in a Ca2+-dependent manner. This clone contains a cDNA insert of 378 base pairs encoding a truncated polypeptide of 126 amino acids that exhibits sequence similarities to the subunits of the mammalian Ca2+/calmodulin-dependent protein kinase II. Northern blot hybridization analysis indicates that this insert hybridizes with a mRNA species of about 2100 nucleotides. Genomic DNA Southern analysis suggests that this mRNA is encoded by a single- or low-copy number gene in apple. Furthermore, CB1-related squences were detected in genomic DNA from other plant species using lambda-CB1 insert as a probe. Our results demonstrate that plant cDNA expression libraries can be successfully screened for genes encoding proteins that bind calmodulin in the presence of Ca2+.