Nucleotide sequencing revealed that gtaB, the structural gene of UDP-glucose pyrophosphorylase (EC 2.7.7.9), is part of a divergon-like genetic entity. The latter consists of two monocistronic operons gtaB and orfX, transcribed from a 245 bp regulatory region, each encoding an acidic protein with a molecular mass of 33.0 and 42.6 kDa, respectively. gtaB is transcribed from a distal PA promoter, and a proximal PB promoter which is negatively controlled by the Sin protein. Sin-mediated transcriptional attenuation and enhancement of PB and PD, respectively, suggest that these promoters control functions which antagonize each other. Transcription of orfX is mediated by a PA promoter. The regulatory region comprises four ATGAAA hexamers, present as two inverse repeats. Protein GtaB exhibits high homology to analogous prokaryotic enzymes, while OrfX shows 55.4% homology with the product of Escherichia coli o389, which is part of a regulatory unit involved in sugar processing. Mutations gtaB515 and gtaBg100, which define different bacteriophage adsorption patterns, were sequenced. They are transitions leading to substitution of amino acids which occupy conserved positions, and are thus likely to be part of an enzyme active site. The nature of the possible receptors for defective bacteriophages PBSY and PBSZ is discussed.