In the presence of sucrose, SacY is activated and prevents premature termination of transcription by binding to a RNA-antiterminator (RAT) sequence (partially overlapping with the terminator sequence) located upstream of the sacB gene. Formation of the SacY-RAT complex prevents alternative formation of the terminator, allowing transcription of the sacB gene. In the absence of sucrose, inhibition of SacY activity by SacX leads to termination of transcription. Phosphorylated by HPr (PtsH). Also phosphorylated by SacX on His-99, leading to its inactivation. Belongs to the transcriptional antiterminator BglG family.