A novel procaryotic transcriptional regulatory element, AlgP, with a histone H1-like carboxy-terminal domain was identified in Pseudomonas aeruginosa. AlgP is required for transcription of the key biosynthetic gene algD, which is necessary for production of the exopolysaccharide alginate causing mucoidy in P. aeruginosa. Mucoidy is a critical virulence determinant of P. aeruginosa invariably associated with the respiratory infections causing high mortality in cystic fibrosis. Here we show that AlgP and histones H1 both have repeated units of the Lys-Pro-Ala-Ala motif (KPAA) and its variations within their long (over 100 amino acids) carboxy-terminal domains. This region of histone H1 tails has been shown to bind to the linker DNA in eucaryotic chromatin fibers. A synthetic 50-mer peptide consisting of repeats from the AlgP carboxy-terminal domain was found to bind DNA in a mobility shift DNA-binding assay. AlgP is encoded by a gene that contains multiple direct repeats organized as tandem, head-to-tail, 12-base-pair (bp) units overlapping with six highly conserved 75-bp units. The repetitive structure of the algP gene appears to participate in the processes underlying the metastable character of mucoidy in P. aeruginosa. Relatively large DNA rearrangements spanning the region with tandem direct repeats encoding the carboxy-terminal histone H1-like structure of AlgP were detected in several strains upon conversion from the mucoid to the nonmucoid phenotype. The frequency of the detectable algP rearrangements associated with the transition into the nonmucoid state varied from strain to strain and ranged from 0 to 50%. The nonmucoid derivatives with the clearly rearranged chromosomal copy of algP were complemented to mucoidy with plasmids containing algP from P. aeruginosa PAO. When a random collection of mucoid strains, isolated from different cystic fibrosis patients, was analyzed by using polymerase chain reaction, an additional level of strain-dependent sequence variation in algP was observed. Variations in the number of the 12-bp repeats were found; however, they did not appear to influence the mucoid status of the strains examined. Thus, the repeated region of algP appears to be a hot spot for DNA rearrangements and strain-dependent variability.