The size and location of the 5' end of celA mRNA encoding endoglucanase A of Clostridium thermocellum were investigated in C. thermocellum and in an Escherichia coli clone that carries and expresses the celA gene. In E. coli, the 5' end of celA mRNA was located 134 bp upstream from the initiation codon and 10 bp downstream from a sequence homologous to the consensus sequence of E. coli sigma 70 and Bacillus subtilis sigma 43 (formerly sigma 55) vegetative promoters. In C. thermocellum, a minor transcript appeared to start from the same site, but a major species started 57 bp upstream from the coding sequence. The 5' end of this mRNA was preceded by a sequence reminiscent of B. subtilis sigma 28 vegetative promoters. In both organisms, the size of the transcript suggested that celA belongs to a monocistronic unit of transcription.