Plays a major role in regulation of the low-calcium response. Seems to sense the calcium concentration and to transmit a signal to shut off yop transcription when the calcium concentration is high. Temperature seems to play the major role in regulation of transcription of the yopN-containing operon of pYV, whereas Ca(2+) concentration has only a moderate effect at 37 degrees C., and no effect at room temperature. Belongs to the HrpJ/YopN family.