Crossover hot-spot instigator (Chi) sequences in Escherichia coli occupy distinct recombination/transcription islands

Crossover hot-spot instigator (Chi) sequences (5'-GCTGGTGG-3') are orientation-dependent, strand-specific sequences implicated in RecA-mediated DNA recombination. In Escherichia coli and Haemophilus influenzae Chi and Chi-like sequences preferentially locate to approx. 1kb recombination 'islands' in the mRNA-synonymous strands of open reading frames (ORFs). Since mRNA-synonymous strands follow Szybalski's transcription direction rule in being G-rich, and the average ORF is about 1kb, then, on this basis alone, Chi sequences are seen to reside in 1kb G-rich 'islands'. However, RecA preferentially binds GT-rich sequences, suggesting that genomic context might potentiate Chi action. Consistent with this, we report for E. coli that 1kb sequence windows with Chi near their centres are a distinct subset of total 1kb windows, the mRNA-synonymous strands being preferentially enriched in both G and T. Chi function might be particularly important for bacteria that survive high temperature and radiation. These often exist in habitats where recombination with E. coli DNA would be unlikely, so canonical Chi sequences might not confer a selective disadvantage in this respect. In general, Chi sequences are not more frequent in thermophilic bacteria and Deinococcus radiodurans, than in E. coli and other mesophilic bacteria. Only two of five thermophilic bacteria examined showed preferential location of Chi sequences to mRNA-synonymous strands. In the thermophile Methanococcus jannaschii, windows containing the canonical Chi sequence do not form a distinct subset. We suggest that in thermophilic bacteria and D. radiodurans the Chi function may be achieved by sequences that differ from the canonical Chi sequence, or that the number of these sequences is sufficient, or that the Chi function is unnecessary.