Transcription of the ileS operon in the archaeon Methanobacterium thermoautotrophicum Marburg

The Staphylococcus aureus ileS gene, encoding isoleucyl-tRNA synthetase, is a member of the T-box family

The Staphylococcus aureus ileS gene, encoding isoleucyl-tRNA synthetase (IleRS), contains a long mRNA leader region. This region exhibits many of the features of the gram-positive synthetase gene family, including the T box and leader region terminator and antiterminator. The terminator was shown to be functional in vivo, and readthrough increased during growth in the presence of mupirocin, an inhibitor of IleRS activity. The S. aureus ileS leader structure includes several critical differences from the other members of the T-box family, suggesting that regulation of this gene in S. aureus may exhibit unique features.

Characterization of Methanobacterium thermoautotrophicum Marburg mutants defective in regulation of L-tryptophan biosynthesis

Three nitrosoguanidine-induced mutants of the archaeon Methanobacterium thermoautotrophicum Marburg resistant to 5-methyltryptophan were isolated and characterized. They were found to take up L-tryptophan, as wild-type cells, via an energy-dependent, low-affinity transport system specific for L-tryptophan, with a Km of 300 microM and a Vmax of 7 nmol/mg (dry weight)/min. Resistance to 5-methyltryptophan was not due to feedback-resistant anthranilate synthase but to constitutive expression of the trp genes, as measured by the specific activities of anthranilate synthase and tryptophan synthase, the enzymes encoded by trpEG and trpB, respectively, of the trpEGCFBAD gene cluster. Estimation of trpE mRNA obtained from mutant cells grown in minimal medium with or without L-tryptophan suggested that constitutive expression resulted from deficient transcriptional regulation. The enhanced expression of the trp genes in the mutants was found to result in intracellular L-tryptophan pools that were two- to fourfold higher than in the wild type. Sequencing of the region upstream of trpE revealed in two mutants point mutations mapping on the 5'-side of the archaeal box A, whereas in the third mutant this region did not differ from that of the wild type. These results suggest that (i) in M. thermoautotrophicum the 5-methyltryptophan-resistant phenotype arises from lesions in components of a regulatory system controlling transcription of the trp genes and (ii) cis-acting sequence elements in front of the trpE promoter may form part of this system.

Regulation of tryptophan biosynthesis in Methanobacterium thermoautotrophicum Marburg

A tryptophan-auxotrophic mutant of the archaeon Methanobacterium thermoautotrophicum Marburg was grown with growth-promoting and growth-limiting concentrations of tryptophan. The specific activities of anthranilate synthase (TrpEG) and tryptophan synthase (TrpB) increased 30- to 40-fold in tryptophan-starved cells. Levels of trpE-specific and trpD-specific mRNAs (transcripts of the first and the last genes, respectively, of the M. thermoautotrophicum Marburg trp gene cluster) increased about 10-fold upon starvation for tryptophan. Thus, the expression of the trp genes appears to be regulated primarily at the level of transcription. These data support transcription of trp genes as an operon and support a regulatory model involving a repressor. Anthranilate synthase was feedback inhibited by L-tryptophan, with a Ki of 3.0 microM. In a leucine-auxotrophic mutant starved for L-leucine, the level of alpha-isopropylmalate synthase (LeuA) was 10-fold higher than in cells grown with L-leucine. In addition to the finding of specific regulation of gene expression by the end products of their respective pathways, it was found that the levels of anthranilate synthase and alpha-isopropylmalate synthase were reduced upon growth in the presence of amino acids of other families, such as L-alanine, L-proline, or L-arginine. Conversely, starvation for tryptophan caused a slight elevation of alpha-isopropylmalate synthase and starvation for leucine caused a significant increase of anthranilate synthase and tryptophan synthase specific activities. The latter effect was also observed at the level of trp-specific mRNA and is reminiscent of general amino acid control.