Structural organization of the transfer RNA operon I of Vibrio cholerae: differences between classical and El Tor strains.

Nine major transfer RNA (tRNA) gene clusters were analysed in various Vibrio cholerae strains. Of these, only the tRNA operon I was found to differ significantly in V. cholerae classical (sixth pandemic) and El Tor (seventh pandemic) strains. Amongst the sixteen tRNA genes contained in this operon, genes for tRNA Gln3 (CAA) and tRNA Leu6 (CUA) were absent in classical strains as compared to El Tor strains. The observation strongly supported the view that the above two pandemic strains constitute two different clones.

Peptidyl transferase activity of tRNA: a quantum chemical study.

The mechanism of protein synthesis is still unknown due to inability to detect the so-called enzyme "peptidyl transferase" even after elucidation of high-resolution crystal structure of ribosome. We have recently shown by model building and semi-empirical energy calculation that the tRNA molecule at P-site of ribosome may act as peptidyl transferase (Das et al. (1999) J. Theor. Biol. 200, 193-205). We proposed that the tetrahedral intermediate formed from nucleophylic attack of CO of P-site amino-acylated tRNA by NH2 of A-site amino-acylated tRNA is converted to a six-member ring intermediate by conformational change. This ring intermediate produces a free tRNA and a tRNA covalently linked to a peptide. However, energy of the six-member ring intermediate was calculated to be quite high. We show here that the energy values of all the reactants, intermediates and products are within the expected range when they are calculated using high level ab initio quantum chemical methods.

Effects of exogenous phytohormones on plastid tRNA modifications in ragi coleoptiles.

The effects of phytohormones on plastid tRNA modifications were investigated in ragi (Eleucine coracana) coleoptiles. Intact 7-day old dark-grown ragi seedlings were given phytohormone, indoleacetic acid (IAA) or isopentenyladenine (i6A) treatment and grown in the dark or under white fluorescent light; coleoptiles were harvested 24 hr following treatment, and plastid total tRNAs were isolated and analyzed for their content of modified nucleotides. A total of 14 modified nucleotides were identified in the total digests of ragi plastid total tRNA preparations; significant increases in the content of some modified nucleotides were observed following treatment of phytohormones in the dark and light. The relative amounts of pT, pm1G, pm7G and pm1A in IAA-treated dark-grown, pi6A, pm2G and pCm in IAA-treated light-grown, and pT and pm2G in i6A-treated light-grown ragi coleoptiles were 2 to 10 times higher than the untreated control coleoptile plastid total tRNA. In order to gain a better understanding of the effects of phytohormones on ragi plastid tRNA modifications, we purified plastid tRNA(Ile)(GAU) from coleoptiles of the aforementioned ragi seedlings and analyzed its modifed nucleotide content. We find that the content of pGm was 4 to 5 times higher in the tRNA(Ile)(GAU) purified from i6A- or IAA-treated dark-grown coleoptiles, and pm7G was 5 to 6 times higher in the tRNA(Ile)(GAU) of i6A-treated light-grown ragi coleoptiles. These results suggest that the synthesis or activity of some plastid-specific tRNA-modifying enzymes may be enhanced by i6A and IAA with two different modes of regulation, one operating in the light and the other operating in the dark.