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In Pseudomonas syringae pv . phaseolicola, Expression of the argK Gene, Encoding the Phaseolotoxin-Resistant Ornithine Carbamoyltransferase, Is Regulated Indirectly by Temperature and Directly by a Precursor Resembling Carbamoylphosphate. Karina López-López, 2004.Pseudomonas syringae pv . phaseolicola synthesizes a non-host-specific toxin, phaseolotoxin, and also synthesizes a phaseolotoxin-resistant ornithine carbamoyltransferase (ROCT) to protect itself from its own toxin . ROCT is encoded by argK, which is expressed coordinately with phaseolotoxin synthesis at 18°C . To investigate the regulatory mechanisms of this system, null mutants were constructed for argK, argF (encoding the phaseolotoxin-sensitive OCTase [SOCT]), and amtA (encoding an amidinotransferase involved in phaseolotoxin synthesis) . The argF mutant did not exhibit arginine auxotrophy when grown in M9 medium at 28°C, because under this condition SOCT was replaced by ROCT . This loss of thermoregulation of argK was apparently caused by accumulation of carbamoylphosphate, one of the substrates of SOCT . Carbamoylphosphate, which has a structure similar to that of the inorganic moiety of phaseolotoxin, was used in induction assays with wild-type P . syringae pv . phaseolicola and was shown to be able to induce argK expression in M9 medium at 28°C . These results indicate that argK expression is independent of temperature and is regulated directly by a compound resembling the inorganic moiety of phaseolotoxin . Identification of a Lycopene ß-Cyclase Required for Bacteriorhodopsin Biogenesis in the Archaeon Halobacterium salinarum. Ronald F. Peck, 2002.Biogenesis of the light-driven proton pump bacteriorhodopsin in the archaeon Halobacterium salinarum requires coordinate synthesis of the bacterioopsin apoprotein and carotenoid precursors of retinal, which serves as a covalently bound cofactor . As a step towards elucidating the mechanism and regulation of carotenoid metabolism during bacteriorhodopsin biogenesis, we have identified an H . salinarum gene required for conversion of lycopene to ß-carotene, a retinal precursor . The gene, designated crtY, is predicted to encode an integral membrane protein homologous to lycopene ß-cyclases identified in bacteria and fungi . To test crtY function, we constructed H . salinarum strains with in-frame deletions in the gene . In the deletion strains, bacteriorhodopsin, retinal, and ß-carotene were undetectable, whereas lycopene accumulated to high levels (  1.3 nmol/mg of total cell protein) . Heterologous expression of H . salinarum crtY in a lycopene-producing Escherichia coli strain resulted in ß-carotene production . These results indicate that H . salinarum crtY encodes a functional lycopene ß-cyclase required for bacteriorhodopsin biogenesis . Comparative sequence analysis yields a topological model of the protein and provides a plausible evolutionary connection between heterodimeric lycopene cyclases in bacteria and bifunctional lycopene cyclase-phytoene synthases in fungi .
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