A family of dispersed repetitive extragenic palindromic DNA sequences in E. coli.

We report the properties of 67 members of a family of dispersed repetitive palindromic extragenic bacterial DNA sequences. These sequences, called palindromic units, appear to be present at least several hundred times outside structural genes on the Escherichia coli chromosome. They are found either in clusters - as in a previously described intercistronic element - or in single occurrences. They are not only found within an operon but also between different operons, including between convergent ones. The palindromic units could yield a stem and loop structure at the level of DNA or RNA. The base of the stem is made of eight remarkably conserved base pairs while the rest varies somewhat in length and sequence. We analyse the data available on the palindromic units and we speculate on their possible roles with emphasis on transcription and mRNA stability or processing, as well as on their possible relation to transposition elements and the modular evolution of the genome.