Distribution and metabolism of ornithine in Rhodopseudomonas spheroides

Abstract

Ornithine lipid was found in chromatophores, in poorly pigmented subcellular fractions from pigmented micro-organisms and in fragments from cells grown under oxygen which have no bacteriochlorophyll. Its quantitative distribution among these different subcellular fractions did not correlate with the distribution of diaminopimelic acid. It is concluded that ornithine lipid is a specific constituent of the cytoplasmic membrane as opposed to the cell wall. Calculations indicate that about 20% of the ornithine lipid in pigmented cells is not associated with chromatophores. The cytoplasmic membrane content of unpigmented cells, calculated on the basis of ornithine lipid as a marker, was 15 to 22% of the total cell protein. Radioactivity from DL-[5- 14 C] ornithine in trace amounts was rapidly incorporated into growing cells. Most of the counts were in proline, arginine and glutamic acid residues of the proteins. However, nearly all the radioactivity incorporated into lipid was still present as ornithine. [5- 14 C]Ornithine incorporated into lipid of oxygen-grown cells did not turn over when the organisms were allowed to adapt to photosynthetic conditions but the lipid from the chromatophores was radioactive. During this adaptation the content of ornithine lipid per cell doubled with respect to the phospholipid, which increased twofold. The time course of these changes was parallel to that of bacteriochlorophyll synthesis. The significance of all these results in relation to the nature and biogenesis of the chromato­phores is discussed. It is pointed out also that studies on the distribution of ornithine lipid in other bacterial species may be of taxonomic importance.