Ureases of Extreme Halophiles of the GenusHaloarculawith a Unique Structure of Gene Cluster

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Abstract
We searched for urease activities in 71 strains of extreme halophiles by a urea-phenol red-agar plate method. Positive strains were further investigated by measuring the ammonia released from urea in cell-free extracts. Only 4 strains of the genus Haloarcula, Har. aidinensis, Har. hispanica, Har. japonica, and Har. marismortui were finally shown as the urease producers. A partially purified urease from Har. hispanica was a typical halophilic enzyme in that it showed maximum activity at 18-23% NaCl and lost the activity irreversibly in the absence of NaCl. Partial genes (1596 bp) of the urease encoding from upstream of the beta subunit down to the N-terminal 139 amino acids of the alpha subunit, were PCR amplified from the four strains, as well as from five urease-negative Haloarcula strains. Strains of other genera, which were urease-negative, did not yield PCR products. The deduced amino acid sequences of the beta subunit and partial alpha subunit were similar to each other (92-100% similarities) and to those from other organisms. Analysis of the draft genome sequence of Har. marismortui, however, suggested that the order of the genes encoding the three subunits (with the total number of amino acids of 834) and four accessory proteins was beta-alpha-gamma-UreG-UreD-UreE-UreF. This order is quite unique, since in other microorganisms the order is gamma-beta-alpha-UreE-UreF-UreG-UreD in most cases. No open reading frames were detected in the PCR-amplified upstream of the beta subunit, suggesting that all Haloarcula species have the same unique structure of the urease gene cluster.
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