Effect of Rate of Replication upon Transcription in Chick Embryo Limb Bud Mesenchyme Cells in Organ Culture

Abstract

In partially synchronized organ cultures of limb bud mesenchyme cells of the 3 1/2-day chick embryo the net accumulation of RNA per cell, including DNA-like RNA, is greater than in randomly dividing cells. Based on levels of newly synthesized DNA, RNA accumulation is greater in the last half of the S period when the rate of DNA synthesis is slower. The ratio of newly synthesized RNA, including DNA-like RNA, to newly.synthesized DNA, is also increased when the rate of DNA replication is reduced with cytosine arabinoside and hydroxyurea. The proportion of newly synthesized non-histone protein to total protein in the chromatin increased with a slower rate of DNA replication and this may account for the augmented transcription. At equivalent levels of inhibition of protein synthesis with the arginine analogue, canavanine, and cycloheximide, it was found that canavanine was more effective in reducing the rate of DNA replication. This suggests that histone synthesis plays a role in controlling the rate of DNA replication. The importance of such findings is that a difference in rate of replication of the same DNA sequences in two cells could cause differential transcription resulting in these cells expressing diverse patterns of cell differentiation.