DNAase hydrolysis of chromatin DNA in intact sea urchin sperm cells. Effect of the ionic strength on the digestion parameters.

Chromatin DNA in intact functional Sphaerechinus granularis sperm cells has been digested with micrococcal nuclease at three different ionic strength conditions. The results show a highly-flexible chromatin organization similar to that found in sperm heads. The rate of digestion, the limit value of acid-soluble material and the fragmentation pattern show that the sensitivity of nucleosome and internucleosome DNA regions to nuclease hydrolysis depends on a delicate balance of polar and non polar interactions. At low ionic strength, both nucleosome and internucleosome regions are rapidly and completely hydrolysed at the same time and a transient subunit fragment of 120 b.p. average length is formed. At high ionic strength, internucleosome regions are preferentially hydrolysed; there is a limit digest value and a stable subunit fragment of 140 b.p. average length is formed. A supernucleosome organization in the high ionic strength environment of the sperm cells is suggested by the transient preferential formation of heptamers of nucleosome DNA fragments.