TRANSCRIPTIONAL ANALYSIS OF THE THREONINE DEHYDROGENASE GENE OF XANTHOMONAS-CAMPESTRIS

The nucleotide sequence has previously been determined for the Xanthom onas campestris pv. campestris gene coding for threonine dehydrogenase (tdh). Flanking this gene are the upstream region possessing promoter activity and the downstream perfect inverted repeat having potential to form a stem-loop structure which resembles a transcription terminat or. In addition, Northern blot analysis suggested the transcript of th is gene to be monocistronic. In the present study, the essential regio n for promoter activity was narrowed down to a stretch of 57 bp which still retained 84% of the promoter activity. The first nucleotide to b e transcribed is the guanosine at 30 nt upstream from the proposed tdh start codon. The putative terminator exhibited transcriptional termin ation activity bidirectionally in both Escherichia coli and X. campest ris. These observations indicate that the transcriptional structure of X. campestris tdh is different from that of E. coli where tdh and kbl are organized into the tdh operon. Furthermore, the expression of tdh in X. campestris is repressed by leucine, a situation different from that in E. coli where leucine induces the expression of tdh operon. (C ) 1997 Academic Press.