Regulation of expression of the transfer genes within Tn916 The thick arrows underneath the figure represent the majority of the transcripts, the thin lines represent lower levels of transcription. The dotted lines represent possible basal level, read-through transcripts. In the absence of tetracycline (Tc), most of the transcripts initiated at Ptet terminate at palorf12, Porf9 transcribes orf9 efficiently, whereas Porf7 directs a low level of transcription through orf7 and orf8. In these conditions, Porf7 and Pxis direct a low level of transcription through the transposition-associated and down stream genes. In the presence of Tc, the transcripts initiated at Ptet read through palorf12, palorf9 and palorf10 which leads to a decreased transcription of orf9 and an increased transcription of orf7 and orf8. The resulting overexpression of orf7 and orf8 stimulates the activity of Porf7, which leads to an increased transcription of the downstream genes. orf9 could repress the activity of Porf7. (Adapted from ref. 60).