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The volatile compounds formed during autoxidation of methyl linolenate (MLe) at 22–24°C were analyzed by a technique which reveals the flavor compounds having the highest flavor units (ratio of the concentration to the odor treshold). After a reaction time of 48 hr, when ca. 20 mol percent of the MLe had been converted into hydroperoxides,trans, cis‐2,6‐nonadienal followed by 1,cis‐5‐octadien‐3‐one,rans,cis‐3‐5‐octadien‐2‐one andcis‐3‐hexenal showed the highest flavor units. After 102 hr 1,cis‐5‐octadien‐3‐one was by far the most important odor compound, followed bycis‐3‐hexenal andtrans,cis‐2,6‐nonadienal.

Aroma dilution analysis of a volatile concentrate isolated from reverted soya-bean oil resulted in 11 odour compounds having high aroma values. Nine of these compounds were identified as: 3(Z)-hexenal,octanal, l-octen-%one, 1,5(Z)-octadien-3-one, nonanal, P(E)-nonenal, 2(Z)-nonenal, 3(Z)nonenal and 2(E),G(Z)-nonadienal. 3(Z)-Hexenal, the Cg-aldehydes and the two vinyl ketones contribute with high aroma values to the beany, grassy odour note of the reversion flavour. Amma-Verschlechterung von Sojaol: Identllizierung intensiver Geruchsstoffe, die bei der Aroma-Reversion entstehen Die Aromaverdunnungsanalyse van einem Konzentrat der fliichtigen Verbindungen aus revertiertem Sojaol ergab 11 Geruchsstoffe rnit hohen Aromawerten. Von diesen Verbindungen wurden neun identifiziert: 3(Z)-Hexenal, Octanal, l-Octen-3-on, 1,5(Z)-actadien-3-on, Nonanal, 2(E)-Nonenal, 2(Z)-Nonenal, B(Z)-Nonenal und 2(E),G(Z)-Nonadienal. 3(Z)-Hexenal, die Cg-Aldehyde und die beiden Vinylketone sind mit hohen Aromawerten an der bohnig, grasigen Geruchsnote des Reversionsgeschmackes beteiligt.

The plasmid pPR20 contains the gene tgt, which encodes tRNA guanine transglycosylase (Tgt), on a 33-kbp DNA insert from a region around 9 min on the Escherichia coli linkage map. The plasmid was subcloned to determine the sequence and organization of the tgt gene. Tgt is a unique enzyme that exchanges the guanine residue with 7-aminomethyl-7-deazaguanine in tRNAs with GU(N) anticodons. After this exchange, a cyclopentendiol moiety is attached to the 7-aminomethyl group of 7-deazaguanine, resulting in the hypermodified nucleoside queuosine (Q). Here we give the complete sequence of a 3,545-bp StuI-BamHI DNA fragment where we found the tgt gene and three previously unknown genes encoding proteins with calculated molecular masses of 42.5 (Tgt), 14, 39, and 12 kDa. The gene products were characterized on sodium dodecyl sulfate gels after synthesis in a combined transcription-translation system. The mRNA start sites of the open reading frames (ORFs) were determined by primer extension analysis. Plasmids containing the ORF encoding the 39-kDa protein (ORF 39) complemented a mutation in Q biosynthesis after the Tgt step. This gene was designated queA. The genes are arranged in the following order: ORF 14 (transcribed in the counterclockwise direction), queA, tgt, and ORF 12 (all transcribed in the clockwise direction). The organization of the promoter sequences and the termination sites suggests that queA, tgt, and ORF 12 are localized on a putative operon together with the genes secD and secF.

An Escherichia coli mutant described by Noguchi et al. [Noguchi, S., et al. (1982) J. Biol. Chem. 275, 6544-6550] contains tRNA lacking the hypermodified wobble nucleoside queuosine (Q) due to an inactive tRNA-guanine transglycosylase (TGT). TGT catalyzes the posttranscriptional base exchange of the Q precursor preQ1 with the genetically encoded guanine in tRNA(Asp,Asn,His,Tyr). The mutant tgt gene was cloned and sequenced; it contained a single point mutation resulting in the change of serine 90 to phenylalanine. Overexpression of the mutant gene yielded TGT(S90F) that showed a reduced solubility and did not purify in the same fashion as the wild-type enzyme. TGT(S90F) has no detectable enzymic activity. To determine whether serine 90 performs a catalytic role in the TGT reaction or whether the loss of activity was caused solely by a conformational change of the enzyme, we used site-specific mutagenesis to construct serine-to-alanine (S90A) and serine-to-cysteine (S90C) mutants. Both S90A and S90C mutants were purified in a manner identical to that used for the wild-type enzyme. SDS-PAGE of dimethyl suberimidate-cross-linked mutants showed a pattern identical to that of the wild-type TGT, indicative of a trimeric quaternary structure. Native PAGE of wild-type and mutant TGTs in the absence and presence of substrate tRNA exhibited band shifts indicating that both mutants retain the ability to bind tRNA.(ABSTRACT TRUNCATED AT 250 WORDS)