Characterization of the plastid-encoded carboxyltransferase subunit (accD) gene of potato.

The plastid accD gene encoding the carboxyltransferase b subunit of acetyl-coenzyme A carboxylase (ACCase) was cloned from potato. Potato accD (saccD) is 2487 bp in length with a 614 bp 5 cent upstream promoter region and an ORF of 1524 bp, corresponding to a polypeptide of 507 amino acids. The N-terminal region lacks recognizable motifs, while the C-terminal regions contains five motifs. Among these is motif II, PLIIVCASGGARMQE, the sole motif present in all available accD sequences of plants and animals, and of E. coli, suggesting that this motif may correspond to the catalytic site. saccD has the typical prokaryotic promoter signatures, TTGACA and TATCAA, which are -35 and -10-like sequences for plastid-encoded RNA polymerase (PEP), at positions -184 and -160, respectively. However, it seems to be transcribed by the nucleus-encoded RNA polymerase because it is expressed in tuber and root, and in the dark (under crippled PEP conditions) and its transcription initiation sites do not correspond to those of PEP. saccD is expressed in all potato tissues, i.e., leaf, stem, root, and tuber, and its transcript is produced at a similar rate in the light and dark, at different developmental stages, and during growth in the presence of different sugars and carbon sources. Taken together, our results suggest that potato accD is a housekeeping gene constitutively expressed in both chloroplast and amyloplast.