Characteristic anticodon sequences of major tRNA species from an extreme thermophile,Thermus thermophilusHB8

Spectinomycin operon of Micrococcus luteus: evolutionary implications of organization and novel codon usage

The complete DNA sequence of the Micrococcus luteus spectinomycin (spc) operon and its adjacent regions has been determined. The sequence has revealed the presence of genes that are homologous to those of the Escherichia coli ribosomal and related proteins, L14, L24, L5, S8, L6, L18, S5, L30, L15, and secretion protein Y (sec Y), and the gene for adenylate kinase (adk). The gene arrangement in the spc operon is essentially the same as that of E. coli except for the absence in the M. luteus spc operon of the genes for S14 and X protein that exist in the E. coli spc operon. SecY and adk seem to be composed of another operon (adk operon) with at least an open reading frame. The deduced amino acid sequences for these ribosomal proteins are well conserved among the two species (40-65% identity). Reflecting the high genomic guanine and cytosine (GC) content of M. luteus (74%), the codon usage of the genes is extremely biased toward use of G and C, about 94% of the codon third positions being G or C. Seven codons, AUA, AAA, AGA, UUA, GUA, CUA, and CAA, all of which have A at the codon third positions, are completely absent in the M. luteus genes examined. Out of 11 genes in the M. luteus spc and adk operons, 5 (10) use GUG (UGA) and 6 (1) use AUG (UAA) as an initiation (termination) codon.

Expression of leucine genes from an extremely thermophilic bacterium in Escherichia coli

The organisation of the leucine genes in Thermus thermophilus HB8 was analysed by examining the ability of recombinant DNAs to complement Escherichia coli mutations. The arrangement of the genes is different from that in the mesophilic bacteria E. coli and Salmonella typhimurium. The promoter responsible for the expression of the leuB, leuC and leuD genes of Thermus HB8 in E. coli was identified. The sequence of Thermus DNA containing this promoter revealed structural similarities to the promoter and attenuator regions of the E. coli leucine operon.

Conformation change of tRNAGlu in the complex with glutamyl-tRNA synthetase is required for the specific binding of L-glutamate

The binding of Thermus thermophilus glutamyl-tRNA synthetase (GluRS) with T. thermophilus tRNAGlu, Escherichia coli tRNAGlu, and amino acids was studied by fluorescence measurements. In the absence of tRNAGlu, GluRS binds with D-glutamate as well as L-glutamate. However, in the presence of E. coli tRNAGlu, GluRS binds specifically with L-glutamate. The KCl effects on the Michaelis constants (Km) for tRNAGlu, L-glutamate, and ATP were studied for the aminoacylation of the homologous tRNAGlu and heterologous tRNAGlu species. As the KCl concentration is raised from 0 to 100 mM, the Km value for L-glutamate in the heterologous system is remarkably increased whereas the Km value for L-glutamate in the homologous system is only slightly increased. The circular dichroism analyses were made mainly of the bands due to the 2-thiouridine derivatives of tRNAGlu in the complex. The conformation change of T. thermophilus tRNAGlu upon complex formation with GluRS is not affected by addition of KCl. In contrast, the heterologous tRNAGlu X GluRS complex is in an equilibrium of two forms that depends on KCl concentration. The predominant form at low KCl concentration is closely related to the small Km value for L-glutamate. In this form of the complex, the conformation of tRNAGlu is appreciably different from that of free molecule. Accordingly, such a conformation change of tRNAGlu in the complex with GluRS is required for the specific binding of L-glutamate as the substrate.