The effect of HMG1 on the transcriptional block caused by left-handed Z-form in a highly negatively supercoiled DNA was examined using a supercoiled plasmid containing a (CG)10 sequence downstream of promoters. The transcription by E. coli RNA polymerase was blocked at the boundary of alternating CG sequence in the Z-form. In the presence of HMG1, RNA polymerase could transcribe through the CG sequence resulting in the chain elongation of transcripts. The addition of HMG1 allowed the stalled RNA polymerase at the CG block to resume transcription. These suggest that HMG1 may remove the Z-block by flipping it back into the B-form.