I would suggest rifampicin, however, that is not for the elongation step.Attached is a paper with a list of RNAP inhibitors some of them have mechanism like Streptolyigin perhaps you will find better luck with finding/synthesizing those.

Article Bacterial RNA Polymerase Inhibitors: An Organized Overview o...

In the past i have used mammalian cells and tested camptothecin. It inhibits the unwinding of DNA and severely limits the elongation and initiation rates of polymerase.

DRB also blocks initiation.

I've screened some inhibitors of transcriptional steps in the past again in mammalian cells... If any of them look interesting I can provide more information.

Article High-throughput single-molecule screen for small-molecule pe...

Hanna Alalam,

Rifampicin can not be suggested in this case. It has completely another function. It blocks ONLY RNA polymerases FREE in the cell. Those attached are still working on the elongation. That means you will have the opposite output from Streptolyigin.

Chun-Chen Yao,

I do not know any other antibiotic but I know Streptolyigin works efficiently.

Check out this paper, its s really helpful:

Chen et al., 2015: Genome-wide study of mRNA degradation and

transcript elongation in Escherichia coli.

Best

Loubna Youssar I did not suggest rifampicin. I just mentioned it since it is most likely to be suggested to target the RNAP, I stated it would not work for the elongation step. The suggested antibiotics are listed in the attached ref and are likely to work similar to Streptolyigin. I apologize if the wording was not very straight forward.

Hanna Alalam, sorry! sometimes fast reading could be misleading. Yes I get the point.