PAO1 grew in defined synthetic medium with any of a broad variety of single sulfur sources, including sulfate, cysteine, thiocyanate, alkanesulfonates, organosulfate esters and methionine, but not with aromatic sulfonates, thiophenols or organothiocyanates or isothiocyanates. During growth with any of these compounds except sulfate, cysteine or thiocyanate, a set of 10 sulfate starvation-induced (SSI) proteins was strongly up-regulated, as observed by two-dimensional protein electrophoresis of total cell extracts. A comparable level of up-regulation was found for the hydrolytic enzyme arylsulfatase, which has previously been used as a marker enzyme for the sulfate starvation response. One of the SSI proteins was identified by N-terminal sequencing as a high-affinity periplasmic sulfate-binding protein, and another was related to thiol-specific antioxidants, but the N-terminal sequences of the other SSI proteins revealed no similarity to N-termini of proteins of known function, and they probably represent uncharacterized enzymes involved in sulfur scavenging when preferred sulfur sources are absent. To study the role that cysteine biosynthetic intermediates play in the synthesis of these proteins , we isolated mini-Tn transposon mutants of with insertions in the and genes, which encode subunits of ATP-sulfurylase and sulfite reductase, respectively. These two genes were cloned and sequenced. showed high similarity to the cognate gene in , whereas encoded a 69.3 kDa protein with two domains corresponding to the CysN and CysC proteins. Sulfate no longer repressed synthesis of the SSI proteins in mutants, but repression was restored by sulfite; in the mutant, sulfate, sulfite and sulfide all led to repression of SSI protein synthesis. This suggests that there are at least two independent corepressors of the sulfate starvation response in this species.


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