A number of examples of circular plasmids with specific functions are known in both prokaryotes and eukaryotes. Several linear plasmids have also been identified, but these are all relatively small: large linear plasmids cannot be separated from chromosomal DNA by conventional techniques. There are several cases where the genetic evidence suggests that a character is encoded by a plasmid but no plasmid can be physically detected. This has been the case for antibiotic synthesis genes in Streptomyces; in particular a plasmid SCP1 in Streptomyces coelicolor has been shown to be involved in methylenomycin production by genetic evidence. We report here the application of orthogonal-field-alternation gel electrophoresis to the isolation of linear plasmids from Streptomyces. We have discovered a large linear plasmid of around 520 kilobases in Streptomyces lasaliensis and subsequently similar giant linear plasmids in other Streptomyces strains. We have confirmed that genes for methylenomycin biosynthesis are located on a series of giant linear plasmids in S. coelicolor. These observations may bear on the genetic variability and unstable genetic character of Streptomyces species.

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The organization of a cloned rRNA gene cluster from Chlorella ellipsoidea chloroplast DNA (cpDNA) has been analyzed. Southern hybridization experiments with labelled chloroplast rRNAs as probes revealed an extraordinarily large size of the 16S-23S rRNA spacer region, ca. 4.8 kbp, almost twice as large as those of most higher plants. The nucleotide sequence determined on this region has shown that: (1) The tRNAIle gene locating in this region is similar to those of higher plant chloroplasts, blue-green algae and E. coli but does not contain any introns in contrast to higher plant chloroplasts.

A 243 bp intron was found within the 23S rRNA gene of the unicellular green alga Chlorella ellipsoidea. This intron is A+T-rich (63.7%) compared with the 23S rRNA (50.5%) and is located in domain II of the 23S rRNA. In contrast to rRNA introns so far known, this intron is considerably small and does not possess features of group I introns in spite of its possible folded secondary structure; this is a new type rRNA intron. The complete nucleotide sequence of the 23S rRNA gene (2,965 bp) was also compared with that of tobacco chloroplasts and E. coli.