Abstract
The in vivo rate of sulfate activation in Penicillium chrysogenum (wild-type strain ATCC 24791) was determined to be 0.19 ± 0.09 μmol g−1 (dry weight) min−1 by the following methods. (i) The maximum growth of the organism in synthetic medium was a linear function of the initial Na2SO4 concentration between 0 and 8 × 10−4 Na2SO4. The growth yield was 1.64 × 10−2 g (dry weight) of mycelium per μmol of added sulfate, corresponding to a minimum sulfur requirement of 61 μmol/g (dry weight). Under these conditions (limiting sulfate) the minimum doubling time of P. chrysogenum in submerged culture was about 3.8 h, corresponding to a maximum exponential growth rate constant of 3.0 × 10−3 min−1. If all the sulfur in this mycelium passed through adenosine-5′-phosphosulfate, the rate of sulfate activation in vivo must have been 0.183 μmol min−1 g−1 (dry weight). (ii) In the presence of excess 35SO42−, the total organic 35S produced varied with the mycelial growth rate. However, until the culture approached maximum density, the product of [(growth rate constant) × (organic 35S content)] was nearly constant at 0.24 to 0.28 μmol min−1 g−1 (dry weight). (iii) A sulfur-starved mycelium pulsed with 10−4 M 35SO42− produced organic 35S at a rate of about 0.10 μmol min−1 g−1 (dry weight) under conditions where the internal concentrations of ATP and sulfate would permit ATP sulfurylase to operate at about 70% of its Vmax. Cell-free extracts of P. chrysogenum growing rapidly on excess sulfate contained 0.22 U of ATP sulfurylase per g (dry weight). Thus, in spite of the relatively low specific activity of homogeneous ATP sulfurylase (0.13 U/mg of protein, corresponding to an active site turnover of 7.15 min−1), the mycelial content of the enzyme was sufficient to account for the observed growth rate of the organism on inorganic sulfate as the sole sulfur source.
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