Mung Bean Nuclease I
II. RESISTANCE OF DOUBLE STRANDED DEOXYRIBONUCLEIC ACID AND SUSCEPTIBILITY OF REGIONS RICH IN ADENOSINE AND THYMIDINE TO ENZYMATIC HYDROLYSIS
Paul Hickok Johnson 1 and M. Laskowski Sr. 1
From the
1 From the Laboratory of Enzymology, Roswell Park Memorial Institute, and the Department of Biochemistry Roswell Park Division, State University of New York at Buffalo, Buffalo, New York 14203
Mung bean nuclease I is highly preferential for single stranded (denatured) DNA; the numerical value of this preference depends on the type of DNA and the reaction conditions; at 37° in the absence of Mg2+ it is 30,000-fold for T4 DNA, 65-fold for crab d(A-T)n, and less than 2-fold for biosynthetic d(A-T)n. No detectable hydrolysis of poly dG·poly dC occurs under these conditions even with a 100-fold excess of enzyme. Addition of mm Mg2+ inhibits the hydrolysis of biosynthetic d(A-T)n 13-fold, but accelerates that of denatured DNA 2- to 3-fold. The temperature coefficient of the reaction in the range 27–37° is 5-fold higher for biosynthetic d(A-T)n than for crab d(A-T)n. With native biosynthetic d(A-T)n conditions exist under which the rate of hydrolysis approaches that of denatured DNA from other sources (T4, Escherichia coli, thymus). Therefore, mung bean nuclease I does not recognize biosynthetic d(A-T)n as a typically double stranded structure. DNA of 
phage which is known to contain an A,T-rich region in the center of the molecule is specifically attacked by mung bean nuclease I at this region.
The most important properties of mung bean nuclease I are (a) its ability in low concentration to remove denatured DNA from a mixture of both forms, and (b) its ability in high concentration to cleave specifically A,T-rich regions of double stranded DNA. The name "region-specific nuclease" is suggested for a new class of enzymes exemplified by mung bean nuclease I.
Submitted on September 26, 1969
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