BIOSYNTHESIS OF DELTA-AMINOLEVULINIC-ACID FROM GLUTAMATE BY SULFOLOBUS-SOLFATARICUS

The extremely thermophilic, obligately aerobic bacterium Sulfolobus so lfataricus forms the tetrapyrrole precursor, delta-aminolevulinic acid (ALA), from glutamate by the tRNA-dependent five-carbon pathway. This pathway has been previously shown to occur in plants, algae, and most prokaryotes with the exception of the a-group of proteobacteria (purp le bacteria). An alternative mode of ALA formation by condensation of glycine and succinyl-CoA occurs in animals, yeasts, fungi, and the alp ha-proteobacteria. Sulfolobus and several other thermophilic, sulfur-d ependent bacteria, have been variously placed within a subgroup of arc haea (archaebacteria) named crenarchaeotes, or have been proposed to c omprise a distinct prokaryotic group designated eocytes. On the basis of ribosomal structure and certain other criteria, eocytes have been p roposed as predecessors of the nuclear-cytoplasmic descent line of euk aryotes. Because aplastidic eukaryotes differ from most prokaryotes in their mode of ALA formation, and in view of the proposed affiliation of eocytes to eukaryotes, it was of interest to determine how eocytes form ALA. Sulfolobus extracts were able to incorporate label from [I-C -14]glutamate, but not from [12-C-14]glycine, into ALA. Glutamate inco rporation was abolished by preincubation of the extract with RNase. Su lfolobus extracts contained glutamate-l-semialdehyde aminotransferase activity, which is indicative of the five-carbon pathway. Growth of Su lfolobus was inhibited by gabaculine, a mechanism-based inhibitor of g lutamate-1-semialdehyde aminotransferase, an enzyme of the five-carbon ALA biosynthetic pathway. These results indicate that Sulfolobus uses the five-carbon pathway for ALA formation.d