A dedicated excision repair pathway, termed transcription-coupled repair (TCR), 
targets the removal of DNA lesions from transcribed strands of expressed genes. 
Transcription arrest at the site of the lesion has been proposed as the first 
step for initiation of TCR. In support of this model, a strong correlation 
between arrest of transcription by a lesion in vitro and TCR of that lesion in 
vivo has been found in most cases analyzed. TCR has been reported for oxidative 
DNA damage; however, very little is known about how frequently occurring and 
spontaneous DNA damage, such as depurination and base deamination, affects 
progression of the transcription complex. We have previously determined that the 
oxidative lesion, thymine glycol, is a significant block to transcription by T7 
RNA polymerase (T7 RNAP) but has no detectable effect on transcription by RNA 
polymerase II (RNAP II) in a reconstituted system with all of the required 
factors. Another oxidative lesion, 8-oxoguanine, only slightly blocked T7 RNAP 
and caused RNAP II to briefly pause at the lesion before bypassing it. Because 
an abasic site is an intermediate in the repair of oxidative damage, it was of 
interest to learn whether it arrested transcription. Using in vitro 
transcription assays and substrates containing a specifically positioned lesion, 
we found that an abasic site in the transcribed strand is a 60% block to 
transcription by T7 RNAP but nearly a complete block to transcription by 
mammalian RNAP II. An abasic site in the nontranscribed strand did not block 
either polymerase. Our results clearly indicate that an abasic site is a much 
stronger block to transcription than either a thymine glycol or an 8-oxoguanine. 
Because the predominant model for TCR postulates that only lesions that block 
RNAP will be subject to TCR, our findings suggest that the abasic site may be 
sufficient to initiate TCR in vivo.