Nucleotide sequence of the replication region of the Nostoc PCC 7524 plasmid pDU1.

The cyanobacteria are an ancient and diverse group of prokaryotes capable of oxygenic photosynthesis and plasmids have been found in both unicellular and filamentous types. Currently, all plasmids are cryptic and only one plasmid, pUH24, from a unicellular non-nitrogen-fixing cyanobacterium, has been sequenced (1). Comparative searches (TFASTA, FASTA (2)) of pUH24 sequences did not show similarity to sequences present in the GenBank data bank (release no. 66). Another plasmid, pDU1, from the filamentous, nitrogen-fixing cyanobacterium Nostoc sp. strain PCC 7524 has been used extensively in a variety of shuttle vectors (2). These vectors replicate in a limited number of filamentous cyanobacteria. The minimum region necessary for replication of this 6.28 kilobasepair (kbp) plasmid has been determined to reside on a 1.75 kbp fragment (4). Interestingly, the same study showed that a 1.3 kbp fragment was sufficient for replication in Anabaena sp. M-131. The nucleotide sequence of the 1.75 kbp replication region of pDU1 is presented here. The sequenced region was isolated from the shuttle vector pRLl (3). A large open reading frame (ORFI) is present from nucleotide position 1260 to 141. This encodes a putative protein of 373 amino acids with a molecular weight of 42,477 Daltons. A putative rho-independent transcriptional termination signal is downstream ofORFI at nucleotide positions 33 to 60. A region of extensive dyad symmetry is located upstream of ORFI at nucleotide positions 1512 to 1546. This region of dyad symmetry is seemingly not necessary for replication in Anabaena sp. M-131. The significance of this region of dyad symmetry is unclear as its presence. is not always correlated with the ability of vector constructs to replicate in various cyanobacteria (4). The presence of ORFI is correlated with the ability of shuttle vectors to replicate in cyanobacteria and thus may encode a protein involved in the plasmid's replication. Comparative searches (FASTA, TFASTA) of the pDUl replication region revealed no sequence similarities in the GenBank or with pUH24 sequences.