The growth of Micrococcus luteus, a soil microorganism that belongs to the high-G؉C gram-positive phylogenetic group, is prevented by bicyclomycin, an antibiotic that inhibits the activity of the M. luteus transcription termination factor Rho. A mutant that can grow in 0.3 mM bicyclomycin has a Rho that is insensitive to bicyclomycin and has the single amino acid residue change of Asp 474 to Gly. These results indicate that the function of its Rho factor is essential for M. luteus and that growth of a gram-positive organism can be blocked by bicyclomycin.Very little is known about the mechanisms for termination of transcription in bacterial organisms other than Escherichia coli. In particular we do not know whether all organisms are like E. coli in having two major mechanisms, one that is intrinsic to the function of RNA polymerase itself and the other dependent on the action of a protein factor called Rho. Bacillis subtilis RNA polymerase is known to terminate transcription spontaneously at sequences that closely resemble intrinsic terminators from E. coli (1). Since B. subtilis is not closely related to E. coli, this result suggests that the intrinsic termination mechanism is likely conserved in the Bacteria. In addition, the genes encoding the subunits of RNA polymerase core from a wide variety of organisms have strongly conserved features (12,13). From this observation we infer that the enzymes in all Bacteria function by the same basic mechanism, a mechanism that would include release of transcripts at intrinsic terminators. As to the prevalence of the Rho-dependent mechanism, genes encoding proteins that would be very similar in structure to E. coli Rho have been found in a variety of highly divergent organisms (10), suggesting that the termination function of Rho is highly conserved. However, a rho homolog is not present in the minimal genome of Mycoplasma genitalium (2), a parasitic pathogen, and functional Rho factors have been isolated from very few organisms.In E. coli, rho is an essential gene. Since efficient termination of transcription at the ends of some operons is known to be dependent on the function of Rho-dependent terminators (11, 14), a likely explanation for the loss of viability in the absence of Rho is that inadvertent transcription of certain sequences of DNA occurs, causing the production of toxic substances. Another possibility is that the cumulative consequences of the inappropriate expression of genes and the synthesis of antisense RNA due to the absence of Rho create a catastrophic situation for the cell.If the efficient function of Rho-dependent terminators is as important in other bacteria as it is in E. coli, a blockage of Rho function in those cells should cause loss of viability. Recently, we isolated Rho factor from Micrococcus luteus (8), an organism that is a member of the high-GϩC gram-positive phylogenetic group and thus is only distantly related to E. coli. We showed that M. luteus Rho does function as a transcription terminator factor in vitro. We also found that it is ...
Phenylobacterium immobile, a bacterium which is able to degrade the herbicide chloridazon, utilizes for L‐tyrosine synthesis arogenate as an obligatory intermediate which is converted in the final biosynthetic step by a dehydrogenase to tyrosine. This enzyme, the arogenate dehydrogenase, has been purified for the first time in a 5‐step procedure to homogeneity as confirmed by electrophoresis. The M r of the enzyme that consists of two identical subunits amounts to 69000 as established by gel electrophoresis after cross‐linking the enzyme with dimethylsuberimidate. The K m values were 0.09 mM for arogenate and 0.02mM for NAD+. The enzyme has a high specificity with respect to its substrate arogenate.
Factor F,, is a nickel-containing coenzyme of methanogenic bacteria with porphinoid structure which is derived from uroporphyrinogen III. It is shown that sirohydrochlorin is metabolized by cell free extracts of Methanobacterium thermoautotrophicum to factor F, demonstrating that this compound, or a reduced form of it, is an intermediate in the biosynthesis of F,, and not only of vitamin B,, and siroheme. Factor F,30Sirohydrochlorin Tetrapyrrole &AminolevuIinic acid Methanobacterium thermoautotrophicum S-Adenosylmethionine
Arogenate dehydrogenase, the terminal enzyme of tyrosine biosynthesis in Streptomyces phaeochromogenes, was purified to homogeneity by a five-step procedure. The enzyme is a dimer of M T 57 600 as determined by dodecyl sulfate polyacrylamide gel electrophoresis after cross-linking of the monomers, or of 66 300 as found by gel permeation chromatography, and consists of two identical subunits ofM T 28 100. The pi of the enzyme is 4.45, and the K m values are O.lOSmM for arogenate and O.OlmM for NAD*. Arogenat-Dehydrogenase aus Streptomyces phaeochromogenes: Reinigung und EigenschaftenZusammenfassung: Die Arogenat-Dehydrogenase katalysiert den letzten Schritt der Tyrosinbiosynthese in Streptomyces phaeochromogenes. Das Enzym wurde in einem funfstufigen Anreicherungsverfahren bis zur Homogenität gereinigt und besitzt folgende Eigenschaften: Es besteht aus 2 identischen Untereinheiten mit einer Molekularmasse von 28.1 kDa. Die Molekularmasse des dimeren Enzyms wurde mittels Gelpermeationschromatographie auf 66.3 kDa und mittels Dodecylsulfat-Polyacrylamid-Gelelektrophorese nach Quervernetzung der Monomeren auf 57.6 kDa bestimmt. Der pl beträgt 4.45. Das Enzym ist spezifisch für seine Substrate Arogensäure und NAD®, und die K mWerte betragen 0.105mM für Arogensäure und O.OlmM für NAD e .
With the discovery of arogenic acid two new pathways for the biosynthesis of phenylalanine and tyrosine have been revealed. The occurrence of two, three, or four pathways for the biosynthesis of phenylalanine and tyrosine in microorganisms and plants may be a useful tool for taxonomic classifications. Investigations on enterobacteriaceae, pseudomonads, flavobacteria, streptomycetes, archaebacteria, and on Sphaerotilus, Trichococcus and Leptothrix species from bulking sludge are described. The possible role of arogenate in the evolution of the pathways for tyrosine and phenylalanine biosynthesis is discussed.
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