Unique presence of 2-methylthio-ribosylzeatin in the transfer ribonucleic acid of the bacterium Pseudomonas aeruginosa

Dynamics and origin of cytokinins involved in plant manipulation by a leaf-mining insect

Several herbivorous insects and plant-associated microorganisms control the phytohormonal balance, thus enabling them to successfully exploit the plant by inhibiting plant defenses and withdrawing plant resources for their own benefit. The leaf-mining moth Phyllonorycter blancardella modifies the cytokinin (CK) profile of mined leaf-tissues, and the insect symbiotic bacteria Wolbachia is involved in the plant manipulation to the benefit of the insect host. To gain a deeper understanding into the possible origin and dynamics of CKs, we conducted an extensive characterization of CKs in larvae and in infected apple leaves. Our results show the enhanced CK levels in mines, both on green and yellow leaves, allowing insects to control their nutritional supply under fluctuating environmental conditions. The spatial distribution of CKs within the mined leaves shows that hormone manipulation is strictly limited to the mine suggesting the absence of CK translocation from distant leaf areas toward the insect feeding site. Mass spectrometry analyses reveal that major CK types accumulating in mines and larvae are similar to what is observed for most gall-inducers, suggesting that strategies underlying the plant manipulation may be shared between herbivorous insects with distinct life histories. Results further show that CKs are detected in the highest levels in larvae, reinforcing our hypothesis that CKs accumulating in the mines originate from the insect itself. Presence of bacteria-specific methylthio-CKs is consistent with previous results suggesting that insect bacterial symbionts contribute to the observed phenotype. Our study provides key findings toward the understanding of molecular mechanisms underlying this intricate plant-insect-microbe interaction.

Isolation of the gene (miaE) encoding the hydroxylase involved in the synthesis of 2-methylthio-cis-ribozeatin in tRNA of Salmonella typhimurium and characterization of mutants

The modified nucleoside 2-methylthio-N-6-isopentenyl adenosine (ms2i6A) is present at position 37 (3' of the anticodon) of tRNAs that read codons beginning with U except tRNA(I,V Ser) in Escherichia coli. Salmonella typhimurium 2-methylthio-cis-ribozeatin (ms2io6A) is found in tRNA, probably in the corresponding species that have ms2i6A in E. coli. The gene (miaE) for the tRNA(ms2io6A)hydroxylase of S. typhimurium was isolated by complementation in E. coli. The miaE gene was localized close to the argI gene at min 99 of the S. typhimurium chromosomal map. Its DNA sequence and transcription pattern together with complementation studies revealed that the miaE gene is the second gene of a dicistronic operon. Southern blot analysis showed that the miaE gene is absent in E. coli, a finding consistent with the absence of the hydroxylated derivative of ms2i6A in this species. Mutants of S. typhimurium which have MudJ inserted in the miaE gene and which, consequently, are blocked in the ms2i6A hydroxylation reaction were isolated. Unexpectedly, such mutants cannot utilize the citric acid cycle intermediates malate, fumarate, and succinate as carbon sources.

Presence of 2-methylthioribosyl-trans-zeatin in Azotobacter vinelandii tRNA

Hydroxylated cytokinin, 2-methylthio-N6-(4-hydroxy-3-methylbut-2-enyl) adenosine, was found in the tRNA of Azotobacter vinelandii. This cytokinin had the trans configuration, unlike the cis configuration reported for that from other bacteria. Culture-condition-dependent changes in the content of this thiocytokinin and a few other thionucleosides in the tRNA of this bacterium have been observed.

The transfer RNA of certain Enterobacteriacae contain 2-methylthiozeatin riboside (ms2io6A) an isopentenyl adenosine derivative

Isopentenyl adenosine derivatives are always located adjacent to the 3' end of the anticodon in transfer RNA and have been implicated in certain biological functions. In the enteric bacterium, E. coli, the derivative is ms2i6A whereas in some plant associated bacteria the derivative is the hydroxylated form, ms2io6A. Anti-i6A immunoadsorbent chromatography has been employed to detect isopentenyl adenosine compounds. In the present study we show that the transfer RNA of three species of enteric bacteria, S. typhimurium, K. pneumoniae, and S. marcescens contains both ms2io6A and ms2i6A. Under the growth conditions utilized the ms2io6A is predominant. The presence of ms2io6A in Enterobacteriacae is particularly noteworthy since in previous work it has been found only in plant-associated species of bacteria.

cis 2-Methylthio-ribosylzeatin (ms2io6A) is present in the transfer RNA of Salmonella typhimurium, but not Escherichia coli

We have identified the cis isomer of N6-(4-hydroxy-isopentenyl)-2-methylthioadenosine (ms2io6A) as a component of the tRNA of Salmonella typhimurium. This is the first report of this compound in the tRNA of any member of the enterobacteriaceae: the nucleoside was previously thought to be found exclusively in plants or plant associated bacteria. Interestingly, all E. coli strains examined were found to lack ms2io6A. Evidence is presented which suggests S. typhimurium tRNA also contains low levels of 5-carboxymethylaminomethyl-2-thiouridine (cmnm5s2U) in addition to 5-methylaminomethyl-2-thiouridine (mnm5s2U).

Distribution of cytokinin-active nucleosides in isoaccepting transfer ribonucleic acids from Agrobacterium tumefaciens

The cytokinin-active isoprenoid nucleosides of Agrobacterium tumefaciens transfer ribonucleic acid were identified by high-pressure liquid chromatography, permethylation, and mass spectroscopy. Besides the expected 6-[(3-methylbut-2-enyl)amino]-9-(beta-D-ribofuranosyl)purine (i6A) and its 2-methylthio derivative (ms2i6A), substantial amounts of cis- and trans-ribosylzeatin (io6A) and cis-2-(methylthio)ribosylzeatin (c-ms2io6A) were present. These hydroxylated side chain derivatives are normally characteristic of plant tRNA. Fractionation of the total bacterial tRNA on BD-cellulose and RPC-5 allowed isolation of purified iso-accepting species whose cytokinin nucleoside contents were then determined. Distribution of the isoprenoid nucleosides among the U-group tRNA species was not uniform. cis-Ribosylzeatin was found almost exclusively in one tRNASer while ms2io6A was found predominantly in tRNAPhe, tRNASer, and tRNATyr. Not all cytokinin-active species were found in every member of the U-group tRNAs. The only species present in tRNATrp was i6A; it contained no zeatin derivatives. The hydroxylation and methylthiolation processes appear to be highly specific and dependent upon tRNA structure or sequence.

Iron-related modification of bacterial transfer RNA

Transfer RNAs isolated from E. coli grown in media where ferric iron is not freely available show well characterized chromatographic changes due to the absence of the methylthio moiety of ms2i6A. The altered tRNA molecules include tRNA trp tRNA tyr, tRNA phe and two minor tRNA ser species. It has been suggested that methylthiolation of tRNA affects its function in regulation. We now show iron-related changes in tRNA trp from S. typhimurium, Ps. aeruginosa and K. pneumoniae. tRNA trp from S. typhimurium contains ms2i6A and it seems probable that the availability of iron affects the synthesis of ms2i6A-tRNA trp from i6A-tRNA trp in this organism. An iron-related methylthiolating system may also be operative in K. pneumoniae. S. marcescens tRNA trp, however was not affected by the availability of iron. Neither ms2i6A nor i6A was found in S. marcescens tRNA, although an, as yet unidentified, hydrophobic nucleoside was present.

Zeatin ribonucleosides in the transfer ribonucleic acid of Rhizobium leguminosarum, Agrobacterium tumefaciens, Corynebacterium fascians, and Erwinia amylovora

Until recently, the presence in transfer ribonucleic acid (tRNA) of the hydroxylated cytokinin ribosylzeatin [N6-(4-hydroxy-3-methylbut-2-enyl)adenosine]was thought to be unique to higher plants. This extension of work from several laboratories indicates the presence of 2-methylthioribosylzeatin in the tRNA of the plant-associated bacteria Rhizobium leguminosarum, Agrobacterium tumefaciens, and Corynebacterium fascians, but not in that of Erwinia amylovora. This cytokinin has the cis configuration, as is normally found in the tRNA's of plants. The tRNA thionucleotide patterns in these bacteria are different from those of Escherichia coli, Bacillus subtilis, and Salmonella typhimurium, which contain the unhydroxylated analogs of ribosylzeatin or 2-methylthioribosylzeatin.

Isolation and identification of cytokinins from Euglena gracilis transfer ribonucleic acid

Three ribonucleosides responsible for cytokinin activity in Euglena gracilis var Bacillaris tRNA have been isolated and identified as 6-(3-methyl-2-butenylamino)-9-beta-D-ribofuranosylpurine, 6-(4-hydroxy-3-methyl-cis-2-butenylamino)-9-beta-D-ribofuranosylpurine, and 6-(4-hydroxy-3-methyl-2-butenylamino)-2-methylthio-9-beta-D-ribofuranosylpurine. The structures of these compounds were assigned on the basis of their chromatographic properties and ultraviolet and mass spectra which were identical with those of the corresponding synthetic compounds. The elution profiles of cytokinin bioassay activity and of 35S radioactivity suggest the presence of a trace amount of 6-(3-methyl-2-butenylamino)-2-methylthio-9-beta-D-ribofuranosylpurine.