The rRNA transcription units in E. coli hold some tRNA genes which are processed and transcribed at the time of rRNA transcription. The other tRNA genes  happen  in  clusters  of  up  to  seven  tRNA  sequences  separated  by  spacer regions. Subsequent transcription by the one prokaryotic RNA polymerase, the

                Figure:  Transcription and processing of tRNA in prokaryotes.

mainly  RNA  transcript  folds  up  into  the  characteristic  stem-loop  structures and is then processed in an ordered series of cleavages by ribonucleases (RNases)  that release and trim the tRNAs to their final lengths. The trimming  and cleavage reactions  at the 5'  and 3'  ends of the precursor  tRNAs  contained RNases D and E, F and P. RNases E, F and P are endonucleases  cutting the RNA within,  whilst  RNase  D is an trimming , exonuclease  the ends of the tRNA molecules.