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J. Bacteriol., Nov 1996, 6419-6426, Vol 178, No. 22
K Carlson, LD Kosturko and AC Nystrom
Synthetic sites inserted into a plasmid were used to analyze the sequence
requirements for in vivo DNA cleavage dependent on bacteriophage T4
endonuclease II. A 16-bp variable sequence surrounding the cleavage site
was sufficient for cleavage, although context both within and around this
sequence influenced cleavage efficiency. The most efficiently cleaved sites
matched the sequence CGRCCGCNTTGGCNGC, in which the strongly conserved
bases to the left were essential for cleavage. The less-conserved bases in
the center and in the right half determined cleavage efficiency in a manner
not directly correlated with the apparent base preference at each position;
a sequence carrying, in each of the 16 positions, the base most preferred
in natural sites in pBR322 was cleaved infrequently. This, along with the
effects of substitutions at one or two of the less-conserved positions,
suggests that several combinations of bases can fulfill the requirements
for recognition of the right part of this sequence. The replacements that
improve cleavage frequency are predicted to influence helical twist and
roll, suggesting that recognition of sequence-dependent DNA structure and
recognition of specific bases are both important. Upon introduction of a
synthetic site, cleavage at natural sites within 800 to 1,500 bp from the
synthetic site was significantly reduced. This suggests that the enzyme may
engage more DNA than its cleavage site and cleaves the best site within
this region. Cleavage frequency at sites which do not conform closely to
the consensus is, therefore, highly context dependent. Models and possible
biological implications of these findings are discussed.
Copyright © 1996, American Society for Microbiology
Short-range and long-range context effects on coliphage T4 endonuclease II-dependent restriction
Department of Microbiology, University of Uppsala Biomedical Center, Sweden. KarinC@bmc.uu.se
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