Comparative Immunological Analyses of the Citric Acid Cycle Enzyme Malate Dehydrogenase from Phototrophic Purple Bacteria

Abstract

Immunological methods (immunodiffusion, immunotitration, and ELISA) were employed to compare the immunological properties of the citric acid cycle enzyme malate dehydrogenase (MDH) from purple nonsulfur bacteria (α and β proteobacteria). The results support previous biochemical studies which showed major structural differences in MDH purified from α and β purple bacteria. Immunological analyses of purple bacterial MDH also lend strong support to the phylogeny of nonsulfur purple bacteria as deduced from 16S ribosomal RNA sequencing. Exceptions, however, were observed in the case of Rhodopseudomonas blastica and Rubrivivax gelatinosus . Despite showing several morphological features consistent with its assignment as a Rhodopseudomonas species, immunological analyses of Rp. blastica MDH indicated that it was much more closely related to Rhodobacter species than to species of Rhodopseudomonas . Ru gelatinosus , a β group species, showed little relationship via MDH analyses to Rhodocyclus species, also β group representatives. It was concluded that at least three structurally and immunologically distinct forms of malate dehydrogenase exist in phototrophic proteobacteria: a form common to all α group members, a form common to β-2 species, and Rubrivivax MDH, which is immunologically distinct from that of all other purple bacterial species.