We have characterized two promoters of the cytochrome oxidase subunit 2 (cox2) gene in Zea perennis mitochondria present in maize lines. Initiation at a site 907 bases upstream of the start codon results in the major approximately 1900 nt cox2 transcript. A sequence just upstream of this site conforms to the consensus described for maize mitochondrial promoters and its transcription is correctly initiated in a maize mitochondrial in vitro transcription extract. A second transcription initiation site (‐347) is used only when the dominant allele of a nuclear gene, Mct, is present and its use results in an additional, shorter major transcript. Sequences flanking the Mct‐dependent transcription initiation site, which we have termed the conditional promoter of cox2 (cpc), do not fit the maize mitochondrial promoter consensus and do not function in the maize in vitro transcription extract. The cpc region does not hybridize with mitochondrial, chloroplast or nuclear DNAs from most maize or teosinte lines. However, the cpc sequence is found in the same position upstream of the cox2 gene in Zea diploperennis mtDNA and it has striking similarity to the previously reported ‘ORF of unknown origin’ fused to the ATPase subunit 6 gene in maize CMS‐C mitochondria. cpc appears to represent a new type of mitochondrial promoter. Further analysis of both conditional and constitutive promoters should help us to better understand the control of transcription in plant mitochondria.

Two DNA sequences were cloned from the genome of cultivated rice (Oryza sativa L.) by cross-hybridization with the human minisatellite sequence 33.6. The rice sequences consisted of tandem direct repeats, which showed significant similarity to the 33.6 consensus sequence. Profiles capable of distinguishing different rice cultivars were detected by cross-hybridization with a DNA probe amplified by the polymerase chain reaction from one of the rice minisatellite sequences.

No abstract

Intergenic spacer fragments from the rDNA repeat unit were isolated from a single accession of each of 9 species that cover the range of genomes found in the Oryza genus (A-F). Seven of the 9 species contained 1 size class of rDNA repeat unit only, while Oryza sativa and Oryza latifolia contained 3 and 2 size classes, respectively, of which fragments were cloned for the major size class only. Oryza australiensis contained an additional BamHI site in the intergenic spacer. Dot blots were prepared and hybridised with a repeat unit from each species. Under high stringency conditions, all probes were specific to species possessing the same genome or genomes.Key words: rDNA, rice, genome-specific, dot blots.