[Mutation induced by space conditions in Escherichia coli strains].

To study the mutagenesis of microorganism under space conditions, three Escherichia coli strains CSH108, A2 and A3, were flown on board the Chinese recoverable satellite (JB1 - B9611020) in Oct . 1996. Three ways of sample boarding were designed . After recovery, survival and frequency of lacI- mutation and Arg+ reversion were determined in the flight strains. The results showed that: E. coli strains could survive after the flight conditions. The lacI- mutation frequency of A3 strain in the biocabin was sixty - seven times that of the ground control, the Arg+ reversion frequency of CSH108 in the lead chamber, was ten times that of the ground control; and among the revertants the mutation frequency of non-sense suppressor increased remarkably. The results indicate that the space conditions may greatly enhance the mutation frequency of certain genes in the microorganism and may provide an effective method for microorganism breeding.

Clustering of modified nucleotides at the functional center of bacterial ribosomal RNA.

An aryl trifluoromethyl diazirine photoreactive derivative was attached to the 2-thiocytidine residue at position 32 of tRNA(IArg) and this derivatized tRNA was bound to Escherichia coli 70S ribosomes. After irradiation at 350 nm the site of cross-linking to the 16S RNA was analyzed by our standard procedures and found to lie within the secondary structural element comprising bases 956-983; this region contains two modified nucleotides at positions 966 and 967. Similarly, an aryl azido photoreactive derivative was attached to the phenylalanine residue of Phe-tRNA(Phe), and the derivatized aminoacyl tRNA was bound to the ribosome either at the A- or the P-site. In both cases, after irradiation at 250 nm, the cross-link site was localized to position 2439 of the 23S RNA; in the secondary structure of the latter the neighboring nucleotide 2442 is base-paired to a modified nucleotide at position 2069. Taken together with other cross-linking data, these results now directly implicate a total of 27 out of the 29 modified nucleotides in E. coli 16S and 23S RNA as lying within or close to the functional center of the ribosome.