Minicircular DNA having sequence homologies with chloroplast DNA in a bleached mutant of Euglena gracilis

'Empty' minicircles and petB/atpA and psbD/psbE (cytb559 alpha) genes in tandem in Amphidinium carterae plastid DNA

Amphidinium carterae minicircle chloroplast DNA was separated from total DNA by centrifugation through a sucrose/NaCl gradient. Sequences of minicircles with psbA and 23S rRNA contained a common region of 67 bp. Primers designed from this generated numerous polymerase chain reaction products of 1.5-2.6 kb. These contained psaA and psaB as one gene/circle, and petB/atpA and psbD/psbE as two genes/circle. 'Empty' minicircles of 1.7-2.5 kb containing no identifiable genes or parts of genes were more abundant than gene-containing circles. From 15 minicircles a minimum common region of 48 bp was identified, with little identity to that from other dinoflagellate minicircles.

Characterization of two circular plasmids from the marine diatom Cylindrotheca fusiformis: plasmids hybridize to chloroplast and nuclear DNA

This paper reports the discovery and initial characterization of two small plasmids, pCf1 and pCf2, in the marine diatom Cylindrotheca fusiformis. Extracted diatom DNA separates into two bands in CsCl-Hoechst 33258 dye gradients. Upon agarose gel electrophoresis of a sample of the upper band of the gradient we observed, in addition to high molecular weight (genomic) chloroplast and mitochondrial DNA, pairs of lower molecular weight bands. These bands contained two species of circular plasmid DNA molecules, as shown by electron microscopy. The nucleotide composition of the plasmids, and chloroplast and mitochondrial DNAs is similar, as indicated by their co-banding in the gradients. They were cloned, and their restriction maps determined, showing that pCf1 is 4.27 and pCf2 4.08 kb in size. By hybridization analysis, we showed that pCf1 and pCf2 share regions of similarity, but not identity. Neither plasmid hybridizes with mitochondrial DNA. Both plasmids hybridize with chloroplast DNA, and pCf2 also hybridizes with nuclear DNA.

Plasmids in diatom species

We have discovered plasmids in 5 of 18 diatom species surveyed. In several species, more than one type of plasmid is present. Several of the plasmids show similarity by hybridization previously characterized plasmids in Cylindrotheca fusiformis (J. D. Jacobs et al., unpublished data). Additionally, there is similarity between the plasmids found in C. fusiformis and chloroplast DNA in three diatom species. These results add to the evidence that the plasmids have features of mobile genetic elements.

Red algal plasmids

Five of 21 red algal genera were found to contain circular dsDNA plasmids, typically of two or more sizes per species. Clones of the two plasmids (GL4.4 and GL3.5 kbp), characterizing all isolates of Gracilariopsis lemaneiformis, do not cross-hybridize with each other, with the nuclear, plastid or mitochondrial genomes of G. lemaneiformis, or with any DNA genomes of the other red algae examined. Clones of both plasmids hybridized with discrete bands on Northern blots of total RNA and poly(A)+ RNA. Sequencing of the G. lemaneiformis 3.5 kbp plasmid revealed two potential open reading frames which, when used to probe Northern blots, confirmed the presence of specific transcripts. These autonomously replicating plasmids are present in high copy number per cell and in constant proportion to each other. Their constancy suggests a function of significance to the species. Red algal plasmids may provide useful vectors for transforming economically important red algal species.

A linear DNA molecule of 5.9 kilobase-pairs is highly homologous to the chloroplast DNA in the green alga Chlamydomonas moewusii

The unicellular green alga Chlamydomonas moewusii contains small DNA species of unknown cellular location. We report that the most abundant of these DNAs, here designated low-molecular-weight DNA (LMW DNA), is a linear molecule of 5.9 kilobase pairs (kbp). Southern blot hybridization and restriction enzyme analysis revealed that the LMW DNA sequence also exists as an integrated sequence in a discrete region of the chloroplast genome. We have confirmed earlier reports that small DNA species related to the LMW DNA are absent from Chlamydomonas eugametos, an alga which is interfertile with C. moewusii. In the C. eugametos chloroplast genome we found only remnants of the LMW DNA sequence.