No abstract

E coli tRNA2Phe was modified at 25 degrees C with 3M sodium bisulphite, pH6.0, for periods of up to 48 hours, Three cytadinine residues, at position 17, 74 and 75 from the 5' end were each deaminated to uridine. The 2-methylthio-N6-isopentenyl adenosine at position 37 formed a 1:1 bi-sulphite addition product which was stable to alkaii. No other residues were permanently modified. The rate of modification of each residue was first order with respect to remaining unmodified nucleotide, the time of half reaction, t1/2, being different for each residue. C17 reaction reacted at twice the rate of cytidine in PolyC, indicating that it occupied a very exposed position in the tRNA.

Escherichiu coli tRNAPh' was modified by 3 M sodium bisulphite pH 6.0 for 24 h in the temperature range 25 "C ( x 5 "C) to 55 "C and in the absence of added magnesium ions. The sites and extents of conversion of cytidines to uridine occurring at each temperature were determined by fingerprinting. The new sites of cytidine modification found at higher reaction temperatures were assumed to arise from breakage of secondary and tertiary structure hydrogen bonds involving cytidine residues. From these data, we conclude that hydrogen bonds within the 'complex core' of the tRNA (including the base pairs G-10 . C-25, C-1 1 G-24 and C-13 . G-21 within the dihydrouridine stem and the tertiary structure base pair G-15 . C-48 melt at a lower temperature than the tertiary structure hydrogen bonds between G-19 in the dihydrouridine loop and C-56 in the TYC loop.Conformational changes in tRNA have been postulated on aminoacylation [l -31 and on enzymatic binding of aminoacyl-tRNA to the ribosome-mRNA complex [4,5]. Certain tRNAs may exist in two different conformations, one active and one inactive in aminoacylation. It is therefore of interest to examine some of the conformations in which tRNA molecules may exist.The detailed X-ray crystallographic analysis of yeast tRNAPh ' [6,7] and accumulated primary sequence data [8] have vastly increased our understanding of the static conformation of tRNAs. However much remains unclear about the nature of the conformational changes which tRNAs could undergo in biochemical processes.The denaturation of tRNA is not a single cooperative process but a sequential one and its study provides a means of assessing the conformational stability of a tRNA. Several such studies have recently been made by physical techniques [9 -171. Chemical modification studies have proved to be an effective technique in the study of the static tertiary structure of tRNAs in solution [18]. In particular, sodium bisulphite has been widely used to convert accessible cytidine residues in tRNAs to uridine residues I19 -251. Here we report an extension of this technique, viz the analysis of E. coli tRNA;he modified with bisulphite at a series of elevated temperatures. We interpret our results in terms of the sequential unfolding of the tRNA and relate our interpretation to those of comparable physical studies. MATERIALS AND METHODS MaterialsSoluble 32P-labelled ribonucleic acid from E. coli K12 CA265 and all other radiochemicals were obtained from the Radiochemical Centre (Arnersham, Bucks). Mixed transfer RNA from E. coli K 12 CA265 was supplied by the Microbiological Research Establishment (Porton, Salisbury, Wilts.). Purified tRNAP'Ie (from E. coli MRE 600) and benzoylated DEAEcellulose were obtained from the Boehringer Corporation, poly(C) was from P-L Biochemicals.E. coli phenylalanyl-tRNA synthetase was prepared from E. coli MRE 600 cells, obtained from the Microbiological Research Establishment, by the method of Stulberg [26], the hydroxyapatite chromatography stages being omitted. Pancreatic ribonuclease, snake...

The synthesis of mitochondrial enzymes induced by exposure of anaerobically grown, lipid-depleted Saccharomyces cerevisiae to oxygen is inhibited by d (−)- threo -chloramphenicol and erythromycin. The concentration of these antibiotics required to cause 50% inhibition of this synthesis is less than 1 m m ; this is also approximately the concentration required to inhibit by the same amount mitochondrial protein synthesis in situ. The synthesis of unsaturated fatty acids, ergosterol, and phospholipid induced by aeration is inhibited by d (−)- threo -chloramphenicol at high concentrations (12 m m ) but is unaffected by erythromycin. l (+)- threo -Chloramphenicol affects neither enzyme nor lipid synthesis and is without effect on mitochondrial protein synthesis in situ. All three compounds inhibit the oxidative activity of isolated mitochondria; the chloramphenicol isomers also inhibit phosphorylation. In a euflavine-derived petite mutant, lacking mitochondrial protein synthesis and respiration, aeration results in the normal development of lipid in the cells, but no synthesis of mitochondrial enzymes. d (−)- threo -Chloramphenicol does not inhibit lipid synthesis in these cells. Thus inhibition of mitochondrial protein synthesis with erythromycin or genetic deletion of mitochondrial protein synthesis results in loss of the capacity to synthesize enzymes during aeration. d (−)- threo -Chloramphenicol, as well as inhibiting induced enzyme formation, inhibits lipid synthesis induced by oxygen. It is unlikely that the latter effect of chloramphenicol is due to inhibition of energy production and transformation, to direct effects on lipid synthesis, or to an inhibition of mitochondrial protein synthesis. It is, however, an effect not shared with the l isomer.