<p> Metabolic assimilation of sulphur from inorganic sulphate, requires sulphate activation by coupling to a nucleoside, for the production of high-energy nucleoside phosphosulphates. This pathway appears to be similar in all prokaryotic organisms. Activation is first achieved through sulphation of sulphate with ATP by sulphate adenylyltransferase (ATP sulphurylase) to produce 5'-phosphosulphate (APS), coupled by GTP hydrolysis. Subsequently, APS is phosphorylated by an APS kinase to produce 3'-phosphoadenosine-5'-phosphosulphate (PAPS) [<cite idref="PUB00015456"/>]. In <taxon tax_id="562">Escherichia coli</taxon>, ATP sulphurylase is a heterodimer composed of two subunits encoded by cysD and cysN, with APS kinase encoded by cysC. These genes are located in a unidirectionally transcribed gene cluster, and have been shown to be required for the synthesis of sulphur-containing amino acids [<cite idref="PUB00015456"/>]. Homologous to this E. coli activation pathway are nodPQH gene products found among members of the Rhizobiaceae family. These gene products have been shown to exhibit ATP sulphurase and APS kinase activity, yet are involved in Nod factor sulphation, and sulphation of other macromolecules [<cite idref="PUB00015457"/>]. With members of the Rhizobiaceae family, nodQ often appears as a fusion of cysN (large subunit of ATP sulphurase) and cysC (APS kinase) [<cite idref="PUB00015457"/>, <cite idref="PUB00015558"/>].</p>