Establishment of transcriptional competence in early and late S phase

In animal cells, the process of DNA replication takes place in a programmed manner, with each gene region designated to replicate at a fixed time slot in S phase. Housekeeping genes undergo replication in the first half of S phase in all cell types, whereas the replication of many tissue specific genes is developmentally controlled, being late in most tissues but early in the tissue of expression(1). Here we employ nuclear DNA injection as an experimental system to test whether this phenomenon is due to differences in the ability to set up transcriptional competence during S phase(2,3). Our results show that, regardless of sequence, exogenous genes are a better template for transcription when injected into nuclei of cells in early as opposed to late S phase, and this expression state, once initiated, is preserved after cell division. DNA injected in late S phase is apparently repressed because it is packaged into chromatin containing deacetylated histones, and the same is true for late replicating chromosomal DNA. These findings suggest a mechanistic connection between replication timing and gene expression that might help to explain how epigenetic states can be maintained in vivo.