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ATP sulfurylase from Escherichia coli K12 catalyzes two, coupled reactions: the hydrolysis of GTP and the formation of activated sulfate (APS). At saturating levels of GTP, the initial rate of APS formation is stimulated 116-fold. The mechanism of this activation has been investigated using isotope trapping, mass spectrometry, and initial velocity kinetic techniques. In the presence of GTP, APS formation proceeds via nucleophilic attack of sulfate at the alpha-phosphoryl group of ATP. Isotope-trapping experiments demonstrate productive, random binding of ATP and GTP. ATP is hydrolyzed to yield AMP and PPi. AMP production requires GTP and is suppressible by sulfate, suggesting GTP-dependent formation of an E*AMP intermediate in the synthesis of APS. Studies using the hydrolysis-resistant nucleotide analogues AMPCPP and GMPPNP demonstrate that GTP hydrolysis precedes scision of the alpha-beta bond of ATP. Product inhibition studies indicate that PPi release occurs prior to the addition of sulfate and APS formation. These results are used to construct a proposed mechanism for the GTP-activated synthesis of APS.

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The genus Halomonas (type species H. elongata) is a new bacterial genus composed of salt tolerant bacteria. The growth characteristics and morphology of the type strain of this bacterial group were studied using both complex and defined media. The salt tolerance of the bacterium decreased significantly on defined medium, suggesting that the type of growth medium used has a great effect on bacterial salt tolerance. Experiments on the effect of temperature on salt tolerance indicate that a temperature of 30 °C permits the greatest salt tolerance. In all of these experiments 0.375 and 1.37 M NaCl yielded the most rapid growth rates while 1.37 and 2.5 M NaCl permitted the greatest temperature tolerance. The Halomonas strain was found to have an absolute requirement for the Na+ cation. While NaNO3 and NaBr would substitute for NaCl in the growth medium, when LiCl, NH4Cl, MgCl2∙6H2O, or KCl was substituted for NaCl, the medium would not support growth. The bacterium consistently retained its rod shape regardless of the NaCl concentration in both complex and defined media. The data indicate that even though this bacterium has been in culture for over 5 years it is still well adapted to life in its natural environment.

The recently described genus Halomonas contains bacteria which are extremely tolerant to salt. The basis for this halotolerance was studied in cells grown in a chemically defined medium containing 0.05, 1.37, or 3.4 M NaCl. The organisms increase the intracellular concentration of sodium and calcium as well as glutamic acid and alanine as the concentration of NaCl in the growth medium increases. The data indicate that in high salt concentrations the osmotic pressure increase associated with those internal changes detected in this study does not balance the changes in the external osmotic pressure.