PORPHOBILINOGEN DEAMINASE IS ENCODED BY A SINGLE-GENE IN ARABIDOPSIS-THALIANA AND IS TARGETED TO THE CHLOROPLASTS

Porphobilinogen deaminase (PBG deaminase) is an early enzyme of the pa thway for chlorophyll and heme synthesis. Using degenerate oligonucleo tide primers, based on amino acid sequence data for purified PBG deami nase from pea, a fragment was amplified from Arabidopsis genomic DNA b y PCR, and then used to isolate both a cDNA and a genomic clone for PB G deaminase from Arabidopsis. The cDNA, shown to be full-length by pri mer extension, encodes a precursor protein of 382 residues, which can be imported into isolated chloroplasts and processed to the mature siz e. The genomic clone encodes an identical sequence to the cDNA, except for the presence of four introns within the coding region of the matu re protein, and 1.7 kb of upstream sequence. There is no obvious TATA box within 50 bp of the transcription start. Southern blot analysis su ggests that PBG deaminase is encoded by a single gene in the Arabidops is genome, and RNase protection experiments demonstrated that this gen e is expressed in both leaves and roots. These results support the con clusion that there is only one form of PBG deaminase in all plant cell s, which is located in the plastid.