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Transcription by RNA polymerase (RNAP) is interrupted by pauses that play diverse regulatory roles. Although individual pauses have been studied in vitro, the determinants of pauses in vivo and their distribution throughout the bacterial genome remain unknown. Using nascent transcript sequencing we identify a 16-nt consensus pause sequence in E. coli that accounts for known regulatory pause sites as well as ~20,000 new in vivo pause sites. In vitro single-molecule and ensemble analyses demonstrate that these pauses result from RNAP/nucleic-acid interactions that inhibit next-nucleotide addition. The consensus sequence also leads to pausing by RNAPs from diverse lineages and is enriched at translation start sites in both E. coli and B. subtilis. Our results thus reveal a conserved mechanism unifying known and newly identified pause events.