| Two fiber cDNA (GhCDKA and GhCycD ) and their corresponding genes have been cloned and characterized. GhCDKA encodes a cyclin-dependent kinase containing 294 amino acids (aa). The kinase contains the conserved A-type CDK PSTAIRE motif in the cyclin-binding domain. Comparison of genomic and cDNA sequences indicates that the GhCDKA coding region is split by 7 introns. Genomic Southern analysis indicates that one or two copies of the GhCDKA gene are present in the cotton genome. Northern blot and RT-PCR data indicate that transcript levels of the GhCDKA gene are high in 5--10 DPA fibers, moderate in 15 and 20 DPA fibers and roots, and low in flowers and leaves. Western analysis reveals that GhCDKA protein levels in fibers increase from 5 to 15 DPA, peak at 15 DPA, and decrease from 15 to 20 DPA. The GhCDKA kinase activity, determined by phosphorylation of histone H1, remains the same during fiber development. Promoter analysis shows that the 5'-flanking region of the GhCDKA gene can direct GUS expression in 5--10 DPA cotton fibers. No phenotypic differences were observed in GhCDKA overexpressed transgenic tobacco plants. The bacterial two-hybrid system reveals that GhCDKA interacts with a number of fiber proteins, including an aldolase, a hydroxyproline-rich glycoprotein, a calmodulin binding protein, a phosphate induced protein, a zinc finger protein, and four hypothetical proteins. The GhCycD gene contains 5 introns and encodes a 380 as protein. The protein contains the conserved LxCxE motif of plant D-type cyclins in the N-terminus. GhCycD has a similar expression pattern as GhCDKA, and was specifically expressed in 5--10 DPA fibers. GhCYCD has been shown to interact with GhCDKA in a bacterial two-hybrid assay. These observations suggest that GhCYCD is a partner of GhCDKA and regulates the kinase activity of the GhCDKA/GhCYCD complex during fiber development. |
