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Incorporation of 13C-labelled glucose, acetate, pyruvate or erythrose allowed the determination of the origin of the carbon atoms of triterpenoids of the hopane series and/or of the ubiquinones from several bacteria (Zymomonas mobilis, Methylobacterium fujisawaense, Escherichia coli and Alicyclobacillus acidoterrestris) confirmed our earlier results obtained by incorporation of 13C-labelled acetate into the hopanoids of other bacteria and led to the identification of a novel biosynthetic route for the early steps of isoprenoid biosynthesis. The C5 framework of isoprenic units results most probably (i) from the condensation of a C2 unit derived from pyruvate decarboxylation (e.g. thiamine-activated acetaldehyde) on the C-2 carbonyl group of a triose phosphate derivative issued probably from dihydroxyacetone phosphate and not from pyruvate and (ii) from a transposition step. Although this hypothetical biosynthetic pathway resembles that of L-valine biosynthesis, this amino acid or its C5 precursors could be excluded as intermediates in the formation of isoprenic units.

Pentacyclic triterpenoids of the hopane family were found in about half of some 100 strains of prokaryotes belonging to diverse taxonomic groups, such a wide distribution indicating the biological significance of these compounds. Hopanoids were found in almost all the cyanobacteria and obligate methylotrophs examined, in all the purple non-sulphur bacteria studied and in many taxonomically diverse Gram-negative or Gram-positive chemoheterotrophs. They were absent in all archaebacteria and purple sulphur bacteria examined as well as in various other Gram-positive or Gram-negative genera. The C30 hopanoids, diploptene and diplopterol, are present in almost all hopanoid-containing prokaryotes. The major compounds are always the C35 bacteriohopanepolyols, which are present at a level of 0-1-2 mg per g dry weight, the most common one being bacteriohopanetetrol. Because of their structural characteristics and their influence on the properties of biological membrane models, these compounds might be the structural equivalents of the sterols found in eukaryotes. 0022-1287/84/0001-1523 $02.00 0 1984 SGM Chlorobium limicola Chlorobium limicola var. 'thiosulfatophilum' 6230 Chromatium vinosum D Ectothiorhodospira mobilis 8 1 15 Ectothiorhodospira shaposhnikovii Moskau N 1 Thiocapsa roseopersicina 63 1 1 Thiocystis violacea 231 1 C. Gram-negative chemoautotrophs and c hemohe terotrop hs Agrobacterium tumefaciens CIP 671 Caulobacter crescentus CIP 771 5 Erwinia herbicola NCIB 9680 Escherichia coli CIP K 1212000 Flexithrix QQ-1 'Methylomonas Clara' Hoechst 'Methylophilus rnethylotrophus' ICI 'Moraxella displex non liquefaciens' CIP 5545 'Moraxella lwoffi' CIP 5382 Paracoccus denitrificans DSM 38 1 -65b Proteus mirabilis CIP A235 Pseudomonas acidovorans ATCC 17046 Pseudomonas aeruginosa ATCC 15692 Pseudomonas chlororaphis ATCC 9446 Pseudomonas diminuta CIP 7 129 Pseudomonas jlmrescens 'Pseudomonas maltophilia' ATCC 17445 Pseudomonas stutzeri ATCC 17588 Rhizobium lupini CIP 6357 Thiobacillus A2 Thiobacillus thioparus NCIB 8370 Xanthomonas campestris CIP 7423 D. Gram-positive bacteria Actinoplanes brasiliensis ATCC 25844 Bacillus subtilist Brevibacterium linens CIP 6372 Clostridium paraputrificum ATCC 25780 Desulfovibrio desulfuricans NCIB 8310 'Micrococcus jlavus' CIP 53 160 Micromonospora sp. Roche 2207-85 Streptococcus faecalis CIP 761 17 'Sporosarcina lutea' CIP 5345 Source and growth conditions

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