We have compared the suppressibility of three isonuclear spontaneous, cytoplasmic petite mutants of Saccharomyces cerevisiae, as measured at three temperatures, 23 degrees C, 28 degrees C and 33 degrees C. The three petites have mitochondrial genomes made up of repeat units which are about 400 bp in size, and carry an origin of replication, ori1. This ori sequence is intact in petite Z1, whereas it lacks GC cluster A in petite 26 and cluster A plus some contiguous nucleotides in petite 14. These deletions lead to the impossibility to form a stem-and-loop structure of the ori sequence, the 'A-B fold', which involves two GC clusters, A and B, and the nucleotides in between. Instead, a 'replacement fold', only involving AT base pairs, is feasible. In petites 14 and 26, suppressivity decreases when the temperature is raised from 28 degrees C to 33 degrees C, and increases when the temperature is lowered from 28 degrees C to 23 degrees C. In contrast, no changes are seen in petite Z1. These temperature effects correlate with the stability of the 'A-B fold' and the instability of the 'replacement folds'. Since suppressibility measures the replicative competitiveness of the petite genome relative to the wild-type genome, these results indicate that an environmental parameter, temperature, can reversibly affect the structure and the functional efficiency of ori sequences in vivo. The evolutionary implications of these findings are discussed.