DIFFERENTIAL EXPRESSION OF THE TOXR REGULON IN CLASSICAL AND EL-TOR BIOTYPES OF VIBRIO-CHOLERAE IS DUE TO BIOTYPE-SPECIFIC CONTROL OVER TOXT EXPRESSION

The two major disease-causing biotypes of Vibrio cholerae, classical a nd El Tor, exhibit differences in their epidemic nature. Their behavio r in the laboratory also differs in that El Tor strains produce two ma jor virulence factors, cholera toxin (CT) and the toxin coregulated pi lus (TCP), only under very restricted growth conditions, whereas class ical strains do so in standard laboratory medium. Expression of toxin and TCP is controlled by two activator proteins, ToxR and ToxT, that o perate in cascade fashion with ToxR controlling the synthesis of ToxT. Both biotypes express equivalent levels of ToxR, but only classical s trains appear to express ToxT when grown in standard medium. In this r eport we show that restrictive expression of CT and TCP can be overcom e in El Tor strains by expressing toxT independently of ToxR. An El To r strain lacking functional ToxT does not express CT or TCP, ruling ou t existence of a cryptic pathway for virulence regulation in this biot ype. These results may have implications for understanding the evoluti on of El Tor strains toward reduced virulence with respect to classica l strains.