REGULATION OF TRYPTOPHAN BIOSYNTHESIS IN METHANOBACTERIUM-THERMOAUTOTROPHICUM MARBURG

A tryptophan-auxotrophic mutant of the archaeon Methanobacterium therm oautotrophicum Marburg was grown with growth-promoting and growth-limi ting concentrations of tryptophan. The specific activities of anthrani late synthase (TrpEG) and tryptophan synthase (TrpB) increased 30- to 40-fold in tryptophan-starved cells. Levels of trp-specific and trpD-s pecific mRNAs (transcripts of the first and the last genes, respective ly, of the M. thermoautotrophicurm 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 trans cription. These data support transcription of trp genes as an operon a nd support a regulatory model involving a repressor. Anthranilate synt hase was feedback inhibited by L-tryptophan, with a K-i of 3.0 mu M. I n a leucine-auxotrophic mutant starved for L-leucine, the level of alp ha-isopropylmalate synthase (LeuA) was 10-fold higher than in cells gr own with L-leucine. In addition to the finding of specific regulation of gene expression by the end products of their respective pathways, i t nas found that the levels of anthranilate synthase and alpha-isoprop ylmalate synthase were reduced upon growth in the presence of amino ac ids of other families, such as L-alanine, L-proline, or L-arginine. Co nversely, starvation for tryptophan caused a slight elevation of alpha -isopropylmalate synthase and starvation for leucine caused a signific ant increase of anthranilate synthase and tryptophan synthase specific activities. The latter effect was also observed at the level of trp-s pecific mRNA and is reminiscent of general amino acid control.