MOLECULAR CHARACTERIZATION OF THE PEND PROTEIN, A NOVEL BZIP PROTEIN PRESENT IN THE ENVELOPE MEMBRANE THAT IS THE SITE OF NUCLEOID REPLICATION IN DEVELOPING PLASTIDS

Plastid nucleoids are known to bind to the envelope membrane in develo ping chloroplasts. Here, plastid DNA is extensively replicated. We pre viously detected a DNA binding protein in the inner envelope membranes of developing plastids in pea and named it PEND (for plastid envelope DNA binding) protein. In this study, we report on the structure and m olecular characterization of a cDNA for the PEND protein. As a result of screening cDNA libraries in lambda gt11 with one of the target sequ ences of the PEND protein as a probe, we obtained a clone (PD2) for a novel DNA binding protein consisting of 633 amino acid residues. Analy sis of the N-terminal sequence of the purified PEND protein indicated that the transit peptide is just 16 residues long. The PEND protein wa s detected specifically in the plastid envelope membrane of young unop ened leaf buds by immunoblot analysis. The PEND protein consists of a basic region plus zipper region, an unprecedented sextuple repeat regi on, and a putative membrane-spanning region. The basic region with a z ipper region seems to have diverged from that of other plant transcrip tion factors. In addition, the PEND protein could be a distant homolog of the trans-Golgi network integral membrane proteins, The PEND prote in is therefore a novel type of DNA binding protein that binds to the membrane as an intrinsic membrane protein.