PRD-containing proteins are bacterial transcriptional antiterminators and activators characterised by a duplicated phosphorylation domain involved in the regulation of catabolic operons. Recent genetic and biochemical studies have suggested how the activity of these regulators is positively or negatively controlled through the multiple phosphorylation of conserved histidyl residues. The regulation mode of these proteins has been examined in light of the recently determined first crystal structure of the phosphorylatable domain of the LicT antiterminator.