A NOVEL SINGLE-STRANDED DNA-SPECIFIC ENDONUCLEASE FROM PEA-CHLOROPLASTS

A nuclease with novel activities has been isolated and purified to app arent homogeneity from pea chloroplasts. The enzyme prefers single-str anded (ss) circular DNA; its activity being 1500-fold higher with the ss circular DNA than with the linear double-stranded DNA substrates. T he single-stranded DNase activity is stable at moderately high tempera ture (50 degrees C) and inhibited in the presence of 75 mM NaCl. It bi nds negatively supercoiled DNA in the stoichiometric fashion, but beha ves catalytically on the single-stranded circular DNA. Although the DN ase activity does not recognize any specific nucleotide sequence, the co-operative mode of activity seems to be a novel one. The protein is a monomer of 35 kDa and binds with DNA predominantly through electrost atic interactions. The drug distamycin blocks the endonuclease activit y suggesting that the protein binds at the minor groove of DNA. A RNas e activity has also been found associated with the ss-DNA endonuclease .