THE CALLUS ASSOCIATED PROTEIN (CAP) GENE OF NICOTIANA-TABACUM - ISOLATION, CHARACTERIZATION, AND EVIDENCE FOR POSSIBLE FUNCTION AS A TRANSCRIPTIONAL FACTOR

We have identified a callus associated protein (CAP) as a new molecula r marker for proliferative growth and early cellular differentiation i n higher plants. Antiserum directed against the Sorghum callus associa ted protein (CAP) was employed to isolate a cDNA clone (CAP-CB) from a n expression library prepared from mRNA from actively dividing cell su spension cultures of Nicotiana tabacum L. cv. xanthi. The derived amin o acid sequence contained structural features of eukaryotic transcript ional factors, including three putative zinc fingers, three activator domains, and a nuclear localization signal. A fusion protein formed be tween part of the CAP-CB polypeptide and Escherichia coli beta-glucuro nidase specifically accumulated within the nuclei of transfected proto plasts, suggesting a nuclear localization for the CAP protein. Increas ed levels of expression of CAP in vivo were observed in actively proli ferating tobacco tissues, whereas expression in vitro was induced when mature, differentiated explants proliferated into callus or cell susp ensions. A 6.2 kb unstable mRNA and two low-abundance nuclear proteins (p66 and p68) containing epitopes encoded by CAP-C2 accumulated in pl ant tissues undergoing proliferative growth but were absent in mature, differentiated tissues. The corresponding genes were also present in Nicotiana sylvestris and Petunia hybrida.