A probable mixed-function supraoperon in Pseudomonas exhibits gene organization features of both intergenomic conservation and gene shuffling

Sequencing of an 8182-bp chromosomal reg-Ion in Pseudomonas stutzeri reveal ed the major portion of an apparent mixed-function supraoperon (defined as a nested organization of transcriptional units encoding gene products which function in more than one biochemical pathway). A nearly identical supraop eron organization was apparent in the unpublished Pseudomonas aeruginosa ge nome database, where the complete Pseudomonas supraoperon was deduced. The serC-(pdxF)-aroQ(p). pheA-hisH(b)-tyrA(c)-aroF-cmk-rpsA supraoperon encodes 3-phosphoserine aminotransferase, a bidomain chorismate mutase/prephenate dehydratase, imidazole acetol-phosphate aminotransferase, cyclohexadienyl d ehydrogenase, 5-enolpyruvylshikimate 3-phosphate synthase, cytidylate kinas e, and ribosomal protein S1. The member genes were identified by homology a nalysis, enzyme assay, and/or functional complementation. Although SerC(Pdx F) and HisR, exercise their primary functions in serine, pyridoxine, and hi stidine biosynthesis, they also have critical catalytic roles in provision of the sidechain amino groups of tryptophan, phenylalanine, and tyrosine. T he Likelihood of supraoperon-wide translational coupling is suggested by th e highly compressed intergenic spacing (including overlapping stop and star t codons), as well as by possible hairpin structures in mRNA which may sequ ester some of the ribosome-binding sites and thus provide a mechanism for t ranslational coupling. A comparison of the organization of the supraoperon genes in other organisms represented in the database revealed unmistakable conservation of the Linkage of these genes across wide phylogenetic boundar ies, albeit with considerable gene shuffling. At least remnants and shuffle d portions of the entire supraoperon are distributed throughout the Gramneg ative bacteria with the hisH(b)-tyrA-aroF gene block being conserved as dis tantly as the gram-positive bacteria. Such conservation of mixed-function g enes may reflect the selective value of still-unknown global relationships of protein-protein interaction or regulation.