Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.2.1.23 (beta-galactosidase)
 14,648 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A series of N-terminal deletions of the mitochondrial targeting sequence of the HEM1 product, delta-aminolevulinate synthase, was constructed. Targeting ability of mutant signals was assayed in vivo by fusion to beta-galactosidase or to the mature delta-aminolevulinate synthase. In the former case, the subcellular location of the fusion proteins provided a measure of import efficiency. In the later case, constructs were tested for complementation of delta hem 1 strains. We found this complementation assay to be particularly sensitive if the delta hem 1 strain is rho-. The results of these experiments, together with experiments deleting the signal from the carboxyl end, indicate that the targeting information is encoded in non-overlapping regions of the HEM1 signal.
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PMID:N-terminal deletions of a mitochondrial signal sequence in yeast. Targeting information of delta-aminolevulinate synthase is encoded in non-overlapping regions. 250 39

delta-Aminolevulinate synthase, the first enzyme in the heme biosynthetic pathway, is encoded by the nuclear gene HEM1. The enzyme is synthesized as a precursor in the cytoplasm and imported into the matrix of the mitochondria, where it is processed to its mature form. Fusions of beta-galactosidase to various lengths of amino-terminal fragments of delta-aminolevulinate synthase were constructed and transformed into yeast cells. The subcellular location of the fusion proteins was determined by organelle fractionation. Fusion proteins were found to be associated with the mitochondria. Protease protection experiments involving the use of intact mitochondria or mitoplasts localized the fusion proteins to the mitochondrial matrix. This observation was confirmed by fractionation of the mitochondrial compartments and specific activity measurements of beta-galactosidase activity. The shortest fusion protein contains nine amino acid residues of delta-aminolevulinate synthase, indicating that nine amino-terminal residues are sufficient to localize beta-galactosidase to the mitochondrial matrix. The amino acid sequence deduced from the DNA sequence of HEM1 showed that the amino-terminal region of delta-aminolevulinate synthase was largely hydrophobic, with a few basic residues interspersed.
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PMID:The nine amino-terminal residues of delta-aminolevulinate synthase direct beta-galactosidase into the mitochondrial matrix. 302 41

Rhodobacter sphaeroides H-5 was isolated as a 5-aminolevulinic acid (ALA) auxotroph following treatment of wild-type cells with N-methyl-N-nitroso-N'-nitroguanidine (J. Lascelles and T. Altshuler, J. Bacteriol. 98:721-727, 1969). The existence in R. sphaeroides 2.4.1 of the genes hemA and hemT, each encoding the enzyme 5-aminolevulinic acid synthase (EC 2.3.1.37), raised questions as to the genetic basis for the ALA auxotrophy in mutant H-5. We therefore cloned both the hemA and hemT genes from mutant H-5. The hemA gene has been sequenced in its entirety and bears four base pair substitutions which encode three amino acid changes relative to the sequence of wild-type strain 2.4.1. Complementation analysis of an Escherichia coli ALA auxotroph has revealed that the loss of ALA synthase activity in the HemA mutant enzyme could be localized to two of the amino acid substitutions. On the other hand, the hemT gene from mutant H-5 was able to complement an E. coli mutant requiring ALA for growth. Complementation analyses were also carried out by introducing the cloned hemA or hemT gene of mutant H-5 or wild-type 2.4.1 in trans into H-5 and, in parallel, into our previously described HemA-HemT double mutant strain AT1 (E. L. Neidle and S. Kaplan, J. Bacteriol. 175:2304-2313, 1993). This analysis revealed that while the complementation pattern of mutant AT1 parallels that for the E. coli ALA auxotroph, mutant H-5 could only be complemented by the wild-type hemA gene. The ability of the hemT gene of either mutant H-5 or wild-type 2.4.1 to complement the ALA auxotrophy of mutant AT1 but not mutant H-5 was consistent with beta-galactosidase activities obtained with hemT-lacZ transcriptional fusions. We conclude that the ALA auxotrophy of mutant H-5 arises from (i) a nonfunctional HemA protein containing multiple missense substitutions and (ii) an inability of the normal hemT gene to be expressed in the mutant H-5 genetic background, i.e., an additional mutation of unknown origin is required for hemT expression. These studies bear directly on the regulation of the expression of the hemA and hemT genes of R. sphaeroides 2.4.1.
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PMID:Regulation of 5-aminolevulinic acid synthesis in Rhodobacter sphaeroides 2.4.1: the genetic basis of mutant H-5 auxotrophy. 775 Dec 86