The Euglena gracilis chloroplast genome: structural features of a DNA region possibly carrying the single origin of DNA replication.

We sequenced a Bg1II-HindIII DNA fragment of the Euglena gracilis chloroplast genome which most likely carries the single origin of DNA replication according to recent electronmicroscopic mapping studies (Koller and Delius 1982a; Ravel-Chapuis et al. 1982). This DNA fragment contains a polymorphic region (Schlunegger et al. 1983) which is composed, as will be shown, of multiple tandem repeats (54 bp, 87% A+T). Furthermore we located on this DNA fragment a short inverted repeat element (96 positions) observed in the electronmicroscopic studies (Koller and Delius 1982b). Between the borders of the polymorphic region and the nearby inverted repeat (distance of 179 positions) we retrieved an exact copy of parts of the rDNA leader (105 positions) including 49 positions of the chloroplast trnW gene. A computer search for bacterial type Ori-regions did not reveal any significant sequence homology. However, the polymorphic region and its immediate vicinity have the capacity to form multiple stem and loop structures which may be involved in DNA replication initiation.