χ*, a χ‐related 11‐mer sequence partially active in an E. coli recC* strain

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Abstract
Background: χ sequence (5′GCTGGTGG) of Escherichia coli was first identified as a site that increased the plaque size of bacteriophage λ. Subsequent studies showed that this site is responsible for both the attenuation of RecBCD exonuclease activity and the promotion of RecA, RecBCD-mediated recombination. It is known that bacteriophage λ containing the χ site makes very small plaques on a recC* (recC1004) mutant because χ is not recognized by the RecBC*D mutant enzyme. Results: We cloned E. coli chromosomal fragments in λ which allowed λ to form larger plaques on this recC* mutant. The fragments were found to share a χ-like 11-mer sequence, 5′GCTGGTGCTCG. Substitution of these fragments with a synthetic 11-mer of this sequence and single-base-pair substitution analysis of its last four nucleotides demonstrated that this sequence is both necessary and sufficient for the observed activity. The sequence, designated χ* (chi-star), protected rolling-circle DNA replication in the recC* mutant and in the recBCD+ strain, most likely because it attenuated the exonuclease activity of the RecBC*D and RecBCD+ enzyme. χ*, did not significantly stimulate λ recombination in two assays. Conclusion: We have discovered that a mutant RecBCD enzyme responds, in vivo, to a longer χ variant.

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