Regulation of isopropylmalate isomerase synthesis in Neurospora crassa

Abstract

The capacity to synthetize isopropylmalate isomerase (EC 4.2.1.33) by Neurospora crassa increased during induction in the presence of cycloheximide but was inhibited by proflavine and other inhibitors of RNA synthesis. Turnover of the enzyme once formed appeared negligible, but the message (measured as enzyme-forming capacity) had a half-life of 4 to 8 min. A comparison of the kinetics of induction in the wild type and a newly isolated alpha-isopropylmalate-permeable strain suggested strongly that feedback control by leucine of alpha-isopropylmalate production can adequately serve as the primary physiological regulator of endogenous inducer concentration. Genetic data are presented which implicate the involvement of two unlinked genes, ipm-1 and ipm-2, in determining permeation of alpha-isopropylmalate.

Evidence that alpha-isopropylmalate synthase of Saccharomyces cerevisiae is under the "general" control of amino acid biosynthesis

“…Regulation of enzyme amount is most pronounced with isopropylrnalate isomerase and ,B-isopropylmalate dehydrogenase, both of which are repressed slightly by leucine and fall to very low levels when both leucine and threonine are present in the growth medium (1, 6). There is also evidence that isomerase and dehydrogenase are induced by a-isopropylmalate (Y.-P. Hsu and G. B. Kohlhaw, unpublished data), possibly in a manner similar to that which is observed in Neurospora crassa (12). Changes in the level of a-isopropylmalate synthase are not as drastic as those seen with the other two enzymes.…”

Specific regulatory interconnection between the leucine and histidine pathways of Neurospora crassa

“…The endogenous levels of aIPM attained by ut+ strains have been found sufficient to induce considerable synthesis of the leucine biosynthetic enzymes, but the endogenous aIPM concentration never reaches a high enough level to replace fully the leucine requirement for growth of such strains. Blocking the conversion of aIPM to leucine by interposing a leu-J or leu-2 mutation in leu4, ut+, however, was found to increase the sensitivity of the pathway to induction (26) and, as indicated in Table 1, also to increase the level of resistance to aminotriazole.…”

“…In analogous fashion, the wild-type strain is considerably more resistant to aminotriazole than are leu-3 and leu4 mutant strains but is more sensitive than the aforementioned nonauxotrophic feedbackinsensitive strain, leu4FLR92. Finally, and perhaps most informative, strains which are permeable to aIPM (designated ut+, a phenotype determined by the two genes ipm-1 and ipm-2 [26]) and produce a functional leu-3 product become more resistant to aminotriazole when aIPM is provided. This response is demonstrated clearly by the multiple mutants leu-J, leu4, ut+ and leu-2, leu4, ut+, as well as by the leu4, ut+ strain itself, in contrast to leu-3, leu4, ut+.…”

“…The kinetics of the increase in IGP dehydratase activity after the addition of the inducer are rather rapid, following closely those of IPM isomerase activity, and require protein synthesis for expression. As a consequence, it seems likely that the effect of aIPM is on the rate of transcription of the his-i gene, as has been shown to be the case for leu-2 (26).…”

Chromosomal loci of Neurospora crassa

“…Able to use a-isopropylmalate to support growth of leu4 mutants and for induction of aisopropylmalate isomerase and P-isopropylmalate dehydrogenase, in contrast to ipm+ strains, which are unable to take up this intermediate (870,871). ipm-2: isopropyhnalate permeation-2 Unmapped.…”