FIG. 3.

Identification of xylF transcript initiation and putative regulatory sequences. (A) Primer extension analysis. The end-labelled primer (5 × 10⁵ cpm) was annealed to RNA (40 μg) from T. ethanolicus grown on 0.4% xylose and was extended with reverse transcriptase. An aliquot from the primer extension reaction (lane x) was electrophoresed on a denaturing 8% polyacrylamide gel along with the sequence ladder generated with the same oligonucleotide (lanes G, A, T, and C). The two cDNA products representing major transcript initiation sites (S₁ and S₂) are marked. (B) Regulatory elements of xylAB and xylF. A portion of the sequence including the xylB 3′ end, xylB-xylF intercistronic spacer, and xylF 5′ terminus is shown along with transcriptional and translational signals. P₁ and P₂ indicate the upstream and downstream xylF promoters, respectively. Long arrows depict a palindromic sequence possibly serving as the xylAB transcriptional terminator and the xylF operator (see Discussion). The potential transcription and translation start sites are shown in boldface. rbs, ribosome-binding sequence; asterisks, translation termination codon of xylB.