HLYM, A TRANSCRIPTIONAL SILENCER DOWNSTREAM OF THE PROMOTER IN THE HLY OPERON OF ESCHERICHIA-COLI

Transcription of the hly operon of transmissible plasmids in Escherich ia coli is subject to a tight regulation which also involves various c hromosomal genes, such as hha. We have identified a 200-bp region with in the hlyC gene, designated hlyM, which modulates hemolysin expressio n, The deletion of hlyM increased the activity of hly::galK fusion 20 fold. hlyM does not contain any internal promoter, nor is it capable o f acting in trans, Our data suggest that the chromosomal Hha protein i nteracts with hlyM in order to silence the hly promoter, In addition, hlyR, a positive activator of hemolysin expression, seems to suppress the modulatory effect dictated by the Hha protein on the hlyM region.