Wild-type cyanobacteria of the genus Anabaena are capable of oxygenic photosynthesis, differentiation of cells called heterocysts at semiregular intervals along the cyanobacterial filaments, and aerobic nitrogen fixation by the heterocysts. To foster analysis of the physiological processes characteristic of these cyanobacteria, we have constructed a family of shuttle vectors capable of replication and selection in Escherichia coli and, in unaltered form, in several strains of Anabaena. Highly efficient conjugative transfer of these vectors from E. coli to Anabaena is dependent upon the presence of broad host-range plasmid RP-4 and of helper plasmids. The shuttle vectors contain portions of plasmid pBR322 required for replication and mobilization, with sites for Anabaena restriction enzymes deleted; cyanobacterial replicon pDUl, which lacks such sites; and determinants for resistance to chloramphenicol, streptomycin, neomycin, and erythromycin.Many filamentous cyanobacteria fix dinitrogen aerobically within specialized cells called heterocysts that differentiate at semiregular intervals along the filaments. All cyanobacteria are capable of oxygenic photosynthesis. Genetic methods usable for study of nitrogen fixation, differentiation, pattern formation, and photosynthesis in these organisms have long been sought.Several unicellular cyanobacteria can be transformed by DNA in the growth medium (1-3). Shuttle vectors, plasmids able to replicate in Escherichia coli and in an alternative host, have been constructed for two such strains, Anacystis nidulans strain R2 and Agmenellum quadruplicatum ). An Anacystis gene cloned in a shuttle vector in E. coli was returned to the cyanobacterium by transformation (7). To date, no reproducible transformation system is known for filamentous cyanobacteria.Conjugation provides an alternative approach to transfer of cloned DNA. RP-4 and related plasmids can transfer themselves or derivatives of themselves into a wide range of We report the construction of a hybrid between pBR322 and plasmid pDU1 (16) from the filamentous cyanobacterium Nostoc. Because restriction endonucleases present in strains of cyanobacteria (17) apparently reduce retention of DNA transferred into those cyanobacteria (4, 18), the hybrid plasmid was restructured to eliminate sites for restriction enzymes present in several strains of Anabaena. Additional antibiotic-resistance determinants were added, lest an organism be unable to use any one such determinant. The derivatives of the hybrid plasmid proved to be shuttle vectors, capable of RP-4-mediated transfer into several strains of Anabaena and of replication in those strains.MATERIALS AND METHODS Anabaena sp. PCC 7120, Anabaena sp. U. Leningrad strain 458 (PCC 7118), Anabaena sp. U. Tokyo M-131, and Nostoc sp. PCC 7524 were grown with nitrate as described (19). Anacystis nidulans strain R2 was grown in medium BG-11 (20).E. coli was grown in L broth, supplemented as appropriate with none, one, or two of the following: 25 jig of chloramphenicol (Cm) per...