The ubiquitous potential Z-forming sequence of eucaryotes, (dT-dG)n . (dC-dA)n, is not detectable in the genomes of eubacteria, archaebacteria, or mitochondria.

The potential Z-forming sequence (dT-dG)n . (dC-dA)n is an abundant, interspersed repeat element that is ubiquitous in eucaryotic nuclear genomes. We report that in contrast to eucaryotic nuclear DNA, the genomes of eubacteria, archaebacteria, and mitochondria lack this sequence, since even a single tract of greater than or equal to 14 base pairs in length is not detectable through either hybridization or sequence analysis. Interestingly, the phylogenetic distribution of the (dT-dG)n . (dC-dA)n repeat exhibits a striking parallel to that of (dT-dC)n . (dG-dA)n, but not to other homocopolymeric sequences such as (dC-dG)n . (dC-dG)n or (dT-dA)n . (dT-dA)n.