Transcription was visualized by inserting upstream stem loops into an endogenous developmentally regulated Dictyostelium gene called dscA. The nascent transcripts were then detected by a GFP fusion protein that binds to the RNA stem loops.
Single-cell analyses revealed discontinuous transcription, or pulses, rather than smooth, uniform expression. Determining the pulse mechanism will require more investigation, but possibilities include reversible chromatin modifications or variations in transcription factor binding.
Pulsing would provide exquisite regulatory control, suggests Singer. “A burst that makes a lot of message might overshoot the amount of protein needed. That can have deleterious effects.” Better to turn on for a flicker, wait to equilibrate, then cycle back on if more protein is needed.
dscA is induced as Dictyostelium form fruiting bodies. This increase did not come from stronger or more frequent pulses, but rather from more pulsing cells. The new recruits were often clustered, suggesting either that the cluster is seeing the same developmental stimulus or that expressing cells tell their neighbors to join in. Whether constitutive genes also pulse remains to be determined.