Prochlorothrix hollandica is an oxygenic photosynthetic prokaryote that differs from the cyanobacteria in having chlorophyll a/b-protein complexes instead of phycobilisomes as major light-harvesting antennae. We report the isolation and culturing of an axenic strain of P. hollandica, available from the Pasteur Culture Collection of Cyanobacteria as strain PCC 9006. The strain has a mean DNA base composition of 51.6 +/- 0.1 mol% G+C and a genomic complexity of 3.37 +/- 0.17 x 10(9) daltons (5,505 kb). A reiterated DNA sequence represents approximately 4.4% of the genome. Restriction enzyme isoschizomers with different sensitivities to base methylation were used to demonstrate that most A residues in the sequence GATC are methylated in P. hollandica DNA and that this methylation increases with culture age. Furthermore, some C residues are methylated, although the specificity of the C methylation system does not match that of well-characterized C methylases. Nucleotide analysis showed that up to approximately 3.5% of both dA and dC residues are methylated in P. hollandica DNA.