Question is rather self-explanatory.
Putting aside other post-transcriptional factors like rate of degradation of transcript, what is the smallest gene ever reported to have successfully been transcribed in E. coli or other similar prokaryotic systems? Why couldn't this gene be any smaller?
Note that in the context of this question, I am disregarding factors like promoters, operators and terminators. Also note that this gene does not necessarily need to code for a protein.