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. Author manuscript; available in PMC: 2009 Feb 14.
Published in final edited form as: Cell Host Microbe. 2008 Aug 14;4(2):134–146. doi: 10.1016/j.chom.2008.06.008

Fig. 1.

Fig. 1

Scheme of system for measuring HIV-1 expression levels under conditions where readthrough transcription can be turned on or off. Using homologous recombination, the HIV-1 genome was inserted into the third intron of the HPRT gene on the X chromosome of the male cell line HCT116. Four clonal cell lines were generated that differed in the orientation of HIV-1 with respect to HRPT transcription (convergent or the same as the host gene HPRT) and on/off status of host gene readthrough due to the presence of a triple polyA signal (Readthrough (+) or Readthrough (−)). Host gene transcription was prematurely terminated by a “stop” site composed of an SV40 triple-polyA signal. The stop signal was flanked by two loxP sites so that it could be removed by Cre/loxP recombination to restore host gene transcriptional readthrough in the Readthrough (+) clones. Solid lines refer to the host gene HPRT, with vertical lines reflecting its exons. The direction of transcription of HPRT and HIV-1 are indicated with black and blue arrows, respectively.