Tan sencillo como eso.

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Note added at 6 mins (2009-02-19 11:02:44 GMT)
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Una explicación en inglés aquí:
http://www.jbc.org/cgi/content/abstract/247/1/305
Gene 6 Exonuclease of Bacteriophage T7
I. PURIFICATION AND PROPERTIES OF THE ENZYME
Carol Kerr 1 and Paul D. Sadowski 1

An exonuclease activity was detectable in crude extracts from Escherichia coli 1200 cells (Endo I-, su-) infected with T7 phage bearing amber mutations in gene 3 (T7 endonuclease I) and gene 5 (T7 DNA polymerase). The activity was not detectable if the infecting phage also contained an amber mutation in gene 6. ***That gene 6 is the structural gene for this enzyme *** was shown by the fact that phage bearing a temperature-sensitive mutation in gene 6 induced an exonuclease activity which was more heat labile than the wild type enzyme.

The enzyme has been purified 500-fold, and the purified preparations are essentially free of ribonuclease, endonuclease, DNA polymerase, and exonuclease I activity although they do contain measurable 3'-phosphatase activity, likely the reflection of residual E. coli exonuclease III.

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Note added at 10 mins (2009-02-19 11:06:07 GMT)
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exonucleasa del gen 6 del bacteriófago T7

http://jvi.asm.org/cgi/content/abstract/44/1/235
Bacteriophage T7 defective in the gene 6 exonuclease promotes site-specific cleavages of T7 DNA in vivo and in vitro.
(...)
These cleavages required the products of genes 3 (endonuclease), 4 (DNA primase), and 5 (DNA polymerase). ***However, the product of gene 6 (exonuclease) must be absent. ***