A new level in the Vibrio cholerae ToxR virulence cascade:: AphA is required for transcriptional activation of the tcpPH operon

The expression of the ToxR virulence regulon is dependent upon the regulatory proteins ToxR/ToxS, TcpP/TcpH and ToxT. We describe here a previously unidentified gene in Vibrio cholerae, aphA ((a) under bar ctivator of tcpP and tcpH expression), which is required for the transcription of the tcpPH operon. Under conditions normally optimal for virulence gene expression, an in frame aphA deletion decreased the expression of a cholera toxin promoter fusion (ctx-lacZ) and prevented the production of the toxin co-regulated pilus (TCP). Plasmids producing ToxT or TcpP/H, but not ToxR, restored ctx-lacZ expression and TCP production in the Delta aphA strain, suggesting that the mutation interferes with toxT expression by influencing the transcription of tcpPH. Indeed, the expression of a chromosomal tcpP-lacZ fusion was reduced in the Delta aphA mutant and increased in both V. cholerae and Escherichia coli by introducing aphA expressed from an inducible promoter. These results support a model in which AphA functions at a previously unknown step in the ToxR virulence cascade to activate the transcription of tcpPH. TcpP/TcpH, together with ToxR/ToxS, then activate the expression of toxT, resulting ultimately in the production of virulence factors such as cholera toxin and TCP.