{HEAD ~ PTGS} depends upon {TAIL ~ transcription} of the initiating transgene to function and so if this gets reduced the PTGS will begin to fail 
depends upon
{ HEAD ~ PTGS } depends upon { TAIL ~ transcription } of the initiating transgene to function and so if this gets reduced the PTGS will begin to fail
HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH