IDENTIFICATION OF THE CHI-SITE OF HAEMOPHILUS-INFLUENZAE AS SEVERAL SEQUENCES RELATED TO THE ESCHERICHIA-COLI CHI SITE

The Escherichia coli Chi site 5'-GCTGGTGG-3' modulates the activity of the powerful dsDNA exonuclease and helicase RecBCD, Genome sequence a nalyses revealed that Chi is frequent on the chromosome and oriented w ith respect to replication on the E. coli genome, Chi is also present much more frequently than predicted statistically for a random 8-mer s equence, Although it is assumed that Chi is ubiquitous, there is virtu ally no proof that its features are conserved in other microorganisms. We therefore identified and analysed the Chi sequence of an organism for which the full genome sequence was available, Haemophilus influenz ae, The biological test we used is based on our finding that rolling c ircle plasmids provide a specific substrate for RecBCD analogues in di fferent microorganisms. Unexpectedly, several related sequences, corre sponding to 5'-GNTGGTGG-3' and 5'-G(G/C)TGGAGG-3', showed Chi activity . As in E. coli, the H. influenzae Chi sites are frequent on the genom e, which is in keeping with the need for frequent Chi sites for dsDNA break repair of chromosomal DNA. Although statistically over-represent ed, this feature is less marked than that of the E. coli Chi site. In contrast to E. coli, the H. influenzae Chi motifs are only slightly or iented with respect to the replication strand. Thus, although Chi appe ars to have a highly conserved biological role in attenuating exonucle ase activity, its sequence characteristics and statistical representat ion on the genome may differ according to the particular features of t he host.