The density and utility of the molecular genetic linkage map of the widespread use of RFLP markers and maps in suncultivated sunflower (Helianthus annuus L.) has been greatly inflower has been restricted by a lack of public RFLP creased by the development and mapping of several hundred simple sequence repeat (SSR) markers. Of 1089 public SSR markers de-probes, consequent lack of a dense public RFLP map, scribed thus far, 408 have been mapped in a recombinant inbred line and low-throughput nature of RFLP markers. The diffi-(RIL) mapping population (RHA280 ϫ RHA801). The goal of the culties posed by the historic lack of public, single-copy present research was to increase the density of the sunflower map by DNA markers were only weakly offset by the emerconstructing a new RIL map (PHA ϫ PHB) based on SSRs, adding gence of facile, universal DNA markers, e.g., RAPDs loci for newly developed SSR markers to the RHA280 ϫ RHA801 RIL (Williams et al., 1990, 1993) and AFLPs (Vos et al., map, and integrating the restriction fragment length polymorphism 1995). RAPDs have primarily been used for tagging (RFLP) and SSR maps of sunflower. The latter was accomplished by phenotypic loci in sunflower, e.g., rust (Puccinia helianadding 120 SSR marker loci to a backbone of 80 RFLP marker loci thi Schw.) and Orobanche cumana Wallr. resistance on the HA370 ϫ HA372 F 2 map. The map spanned 1275.4 centimorgans (cM) and had a mean density of 6.3 cM per locus. The genes (Lawson et al., 1998; Lu et al., 2000). While RAPD PHA ϫ PHB SSR map was constructed from 264 SSR marker loci, and AFLP markers have a multitude of uses, both are spanned 1199.4 cM, and had a mean density of 4.5 cM per locus. The dominant, multicopy, and often nonspecific in nature RHA280 ϫ RHA801 map was constructed by adding 118 new SSR and, as a whole, unsatisfactory for establishing a geand insertion-deletion (INDEL) marker loci to 459 previously nome-wide framework of DNA markers for anchoring mapped SSR marker loci. The 577-locus map spanned 1423.0 cM and cross referencing genetic linkage maps. Single-copy, and had a mean density of 2.5 cM per locus. The three maps were codominant DNA markers, e.g., SSRs, are preferred for constructed from 1044 DNA marker loci (701 unique SSR and 89 such purposes and, until recently, have been lacking unique RFLP or INDEL marker loci) and supply a dense genomein sunflower. wide framework of sequence-based DNA markers for molecular breeding and genomics research in sunflower.

5-Fluorouracil is readily incorporated into active tRNA(Val) transcribed in vitro from a recombinant phagemid containing a synthetic E. coli tRNA(Val) gene. This tRNA has the expected sequence and a secondary and tertiary structure resembling that of native 5-fluorouracil-substituted tRNA(Val), as judged by 19F NMR spectroscopy. To assign resonances in the 19F spectrum, mutant phagemids were constructed having base changes in the tRNA gene. Replacement of fluorouracil in the T-stem with cytosine, converting a FU-G to a C-G base pair, results in the loss of one downfield peak in the 19F NMR spectrum of the mutant tRNA(Val). The spectra of other mutant tRNAs having guanine for adenine substitutions that convert FU-A to FU-G base pairs all have one resonance shifted 4.5 to 5 ppm downfield. These results allow assignment of several 19F resonances and demonstrate that the chemical shift of the 19F signal from base-paired 5-fluorouracil differs considerably between Watson-Crick and wobble geometry.

The use of molecular markers to facilitate the introgression of plant introduction (PI) germplasm into elite soybean [Glycine max (L.) Merr.] cultivars will depend on the amount of polymorphism that exists between elite genotypes and PIs. The objective of this study was to assess the simple sequence repeat (SSR) diversity of 39 elite soybean genotypes (Elites) and 40 PIs that were selected for high yield potential. A total of 397 alleles were detected among the 79 genotypes at 74 SSR marker loci. The number of alleles detected among the PIs was 30% greater than that detected among the Elites. There were 138 alleles specific to the PIs that occurred across 60 SSR loci and 32 alleles specific to the Elites that occurred across 27 SSR loci. Average marker diversity among the PIs was 0.56 and ranged from 0.0 to 0.84. Average marker diversity among the Elites was 0.50 and ranged from 0.0 to 0.79. Genetic similarity estimates based on simple matching coefficients revealed more genetic diversity among the PIs than among the Elites. The greatest genetic diversity was between the PIs and Elites. The ability of SSRs to distinguish among elite soybean genotypes and PIs with agronomic merit may assist with the transfer of favorable alleles from PIs into elite soybean cultivars.

We have studied the interactions between Escherichia coli tRNAVal and valyl-tRNA synthetase (ValRS) by enzymatic footprinting with nuclease S1 and ribonuclease V1, and by analysis of the aminoacylation kinetics of mutant tRNAVal transcripts. Valyl-tRNA synthetase specifically protects the anticodon loop, the 3' side of the stacked T-stem/acceptor-stem helix, and the 5' side of the anticodon stem of tRNAVal against cleavage by double- and single-strand-specific nucleases. Increased nuclease susceptibility at the ends of the anticodon- and T-stems in the tRNAVal.ValRS complex is indicative of enzyme-induced conformational changes in the tRNA. The most important synthetase recognition determinants are the middle and 3' anticodon nucleotides (A35 and C36, respectively); G20, in the variable pocket, and G45, in the tRNA central core, are minor recognition elements. The discriminator base, position 73, and the anticodon stem also are recognized by ValRS. Replacing wild-type A73 with G73 reduces the aminoacylation efficiency more than 40-fold. However, the C73 and U73 mutants remain good substrates for ValRS, suggesting that guanosine at position 73 acts as a negative determinant. The amino acid acceptor arm of tRNAVal contains no other synthetase recognition nucleotides, but regular A-type RNA helix geometry in the acceptor stem is essential [Liu, M., et al. (1997) Nucleic Acids Res. 25, 4883-4890]. In the anticodon stem, converting the U29:A41 base pair to C29:G41 reduces the aminoacylation efficiency 50-fold. This is apparently due to the rigidity of the anticodon stem caused by the presence of five consecutive C:G base pairs, since the A29:U41 mutant is readily aminoacylated. Identity switch experiments provide additional evidence for a role of the anticodon stem in synthetase recognition. The valine recognition determinants, A35, C36, A73, G20, G45, and a regular A-RNA acceptor helix are insufficient to transform E. coli tRNAPhe into an effective valine acceptor. Replacing the anticodon stem of tRNAPhe with that of tRNAVal, however, converts the tRNA into a good substrate for ValRS. These experiments confirm G45 as a minor ValRS recognition element.

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