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Title
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Proteins encoded by the DpnII restriction gene cassette. Two methylases and an endonuclease.
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Authors
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A.G.de la Campa,
P.Kale,
S.S.Springhorn,
S.A.Lacks.
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Ref.
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J Mol Biol, 1987,
196,
457-469.
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PubMed id
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Abstract
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Proteins encoded by three genes in the DpnII restriction enzyme cassette of
Streptococcus pneumoniae were purified and characterized. Large amounts of the
proteins were produced by subcloning the cassette in an Escherichia coli
expression system. All three proteins appear to be dimers composed of identical
polypeptide subunits. One is the DpnII endonuclease, and the other two are DNA
adenine methylase active at 5' GATC 3' sites. Inactivation of enzyme activity by
insertions into the genes and comparison of the DNA sequence with the
amino-terminal sequence of amino acid residues in the proteins demonstrated the
following correspondence between genes and enzymes. The promoter-proximal gene
in the operon, dpnM, encodes a 33 X 10(3) Mr polypeptide that gives rise to a
potent DNA methylase. The next gene, dpnA, encodes the 31 x 10(3) Mr polypeptide
of a weaker and less-specific methylase. The third gene, dpnB, encodes the 34 x
10(3) Mr polypeptide of the endonuclease. Although the endonuclease polypeptide
is initiated from an ordinary ribosome-binding site, each of the methylase
polypeptide begins at an atypical site with a consensus sequence entirely
different from that of Shine & Dalgarno. This presumptive novel
ribosome-binding site is well recognized in both S. pneumoniae and E. coli.
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