Expression and subcellular location of the tetrapyrrole synthesis enzyme porphobilinogen deaminase in light-grown Euglena gracilis and three nonchlorophyllous cell lines.

The expression and subcellular location of porphobilinogen deaminase (PBGD, also known as hydroxymethylbilane synthase; EC 4.3.1.8), one of the early enzymes of porphyrin synthesis, was investigated in light-grown Euglena and in three cell lines that do not contain chlorophyll: dark-grown Euglena, a streptomycin-bleached mutant, and Astasia longa. In wild-type Euglena, immunogold electron microscopy demonstrated that all the immunodetectable enzyme protein was in the chloroplast. PBGD was shown to be photoregulated, and like many other nuclear-encoded proteins in Euglena, the regulation was at the posttranscriptional level. In the three nonchlorophyllous cell lines, as in light-grown Euglena, a single protein of 40 kDa was detected with antiserum to PBGD. This same antiserum immunoprecipitated a larger precursor protein from the total translation products of poly(A)+ RNA, and a single transcript, which was large enough to encode the precursor, was detected on Northern blots of all four cell types. Therefore, in cells that make chlorophyll and those that do not (or cannot), PBGD is located in the plastid. No evidence was obtained for another form of the enzyme, which suggests that in Euglena there is only one pathway for the synthesis of the tetrapyrrole moiety of chlorophyll and heme.