Histone content of germinating pea embryo chromatin decreases as DNA replicates

Abstract

MODIFICATIONS in the expression of genetic information in eukaryotes are probably controlled by changes in the proteins associated with chromatin¹. Unfortunately, in most of the differentiating systems which have been studied these changes are barely detectable, so that we do not know either which changes occur or when they occur. As a part of a more general study of the molecular developmental biology of seed germination we have investigated changes in chromatin of germinating pea embryo axes. Some particularly interesting features of this biological system make it useful for studying developmental changes in chromatin structure. First, it is quite easy to obtain large numbers of homogeneous seeds. Second, germination constitutes a switch from a dormant, arrested state to a new developmental programme. It is thus possible to study the sequence of molecular events which underlie the transition from a completely inactive genome to an active one. Third, this reactivation of the genome is accompanied by two waves of synchronous mitoses which can be considiered as temporal markers. Finally, the germinating embryo constitutes a set of differentiating tissues. To our knowledge, no animal system shares all these features together. The results which are described in the following sections demonstrate that a considerable loss of histone per DNA unit is associated with this reactivation of the genome during germination and that most of these changes take place as DNA replicates.