1. Extracts of Escherichia coli A.T.C.C. 9723 and K(12)703 contain serine transacetylase and O-acetylserine sulphhydrase. Synthesis of the latter enzyme is repressed by growth on l-cyst(e)ine and other sulphur compounds. 2. O-Acetyl-l-serine added to cells growing on glutathione or sulphate as source of sulphur induces the enzymes that catalyse (a) the activation of sulphate to adenosine 3'-phosphate 5'-sulphatophosphate (EC 2.7.7.4 and 2.7.1.25), (b) the reduction of adenosine 3'-phosphate 5'-sulphatophosphate to sulphite and (c) the reduction of sulphite to sulphide (EC 1.8.1.2). Hydrogen sulphide is liberated from cultures growing on sulphate as source of sulphur and in the presence of O-acetylserine. 3. The cysE mutants of E. coli K(12) lack serine transacetylase. Addition of O-acetylserine permits growth on sulphate as source of sulphur; at the same time the enzymes of sulphate reduction, previously absent, are synthesized. Such mutants have no detectable intracellular cyst(e)ine when starved of sulphur. 4. These results suggest that O-acetylserine is necessary for synthesizing the enzymes of sulphate reduction in E. coli. Its action does not appear to be by interference with the repressive control exerted over these enzymes by cyst(e)ine.

The sori of Dictyostelium discoideum (strains SGI, SGZ, NC4 and V12) contained more than 100 mM ammonium phosphate. Glutamine synthetase (GS), which could remove ammonia from the sorus, was not present in 2-d-old dormant spores but enzyme activity returned to vegetative levels after spore germination. Based on mRNA blotting, the activity of this enzyme in germinating spores appeared to be transcriptionally controlled. A t the same time that GS activity was increasing, ammonia was released from germinating spores. Exogenous ammonium ions a t a concentration of 28 mM did not block germination nor modulate GS activity in nascent amoebae. It was concluded that the transcription and translation of GS is not environmentally regulated but is an integral part of the germination process, preparing nascent amoebae for vegetative growth. An exogenous concentration of 69 mM ammonium phosphate could maintain dormancy in spores of strains SGI and SG2 for a t least a week in the absence of any other inhibitory component from the sori. The inhibition was reversible a t any time either by dilution or by washing the spores free of the ammonium ion. Spores of strain acg-were not inhibited by 100 mM ammonium phosphate. A model is presented in which GS in prespore cells serves as a sink for ammonia to allow the osmotically sensitive adenylyl cyclase aggregation protein (ACA) to activate protein kinase A (PKA) to induce fruiting-body formation. After fruiting-body formation is complete, the decline in GS and ACA activities in developing spores is offset by their replacement with the osmotically and ammonia-stimulated adenylyl cyclase osmosensor for germination (ACG). Ammonia and discadenine may act as separate signals to synergistically activate PKA by stimulating ACG activity while inhibiting CAMP phosphodiestrase activity in fully dormant spores.

1. The intracellular cysteine and glutathione concentrations were measured in Escherichia coli under a variety of growth conditions. 2. An inverse relation between intracellular cysteine concentration and the specific activity of the sulphate-activating enzymes was found. 3. This is compatible with the view that the intracellular cysteine concentration controls the rate of synthesis of these enzymes.

1. ATP-sulphate adenylyltransferase (EC 2.7.7.4) and ATP-adenylyl sulphate 3'-phosphotransferase (EC 2.7.1.25) of Escherichia coli 9723, E. coli K(12) and Bacillus subtilis 1379 are each repressed by growth in the presence of cystine. Repression of the two enzymes in E. coli 9723 may be co-ordinate. 2. ATP-sulphate adenylyltransferase of Desulphovibrio desulphuricans, in which sulphate reduction is linked to the energy supply of the organism, is not repressed by growth in the presence of inorganic sulphite or cysteine. 3. Leuconostoc mesenteroides lacks all the enzymes between sulphate and cysteine whether grown on cysteine or glutathione.

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