It was shown with the use of specific probes that mild
micrococcal nuclease digestion released from
chromatin actively-transcribed genes as small
nucleosome oligomers. In the present work we demonstrate that most if not all of the active genes are accessible to the nuclease. It was found that the short released fragments are greatly enriched in transcribed DNA sequences, the most enriched being the dimers of
nucleosomes since 35% of their
DNA could be hybridized to cytoplasmic
RNA. The results of
cDNA-
DNA hybridizations indicate that the monomers and dimers of
nucleosomes contain most of the DNA sequences which encode
poly(A+) RNAs, however larger released fragments include some transcribed sequences, while the nuclease resistant
chromatin is considerably impoverished in coding sites. These evidences are the finding that about 25% of the
DNA from the dimers of
nucleosomes are exclusively located in this class of fragments, tend to prove that the active
chromatin regions are attacked in a non-random way by
micrococcal nuclease. We have previously isolated, without using exogenous nuclease, an actively transcribed genomic fraction amounting to 1.5-2% of the total nuclear
DNA, formed of
single-stranded DNA. In the present study we show that all or nearly all the
single-stranded DNA sequences could be reassociated with the
DNA fragments present in the released monomers and dimers of
nucleosomes. Our observations confirmed our previous finding that the greatest part of
single-stranded DNA selectively originates from the coding strand of genomic
DNA.