Molecular characterization of two large DNA plasmids in the red alga Porphyra pulchra

Five plasmids occur in the red alga Porphyra pulchra. The two larger ones (6859 and 6427 bp) differ in their sequences. The three smaller (1896, 2100, and 2102 bp) have sequences that are similar to one another. These plasmids are circular, double-stranded DNA, present in high copy number, and maintained in this organism through successive generations of laboratory culture. Sequence analysis of the two larger plasmids reveals few prominent structural features, but several potential open reading frames (ORFs) occur, some of which are transcriptionally active. Sequence database comparisons find significant sequence similarity between ORF3 or PP6427 and a 411 amino-acid polypeptide previously characterized in plasmid GC2 from the red alga Gracilaria chilensis. These data support the presence of at least one conserved plasmid coding region in distant orders of red algae. Southern blots of total genomic DNA from other red algae probed with plasmids from P. pulchra demonstrate no hybridization to previously studied Gracilariales species but notable hybridization to several species within the genus Porphyra, although the sizes of detected bands vary.