Regions 2,3 and 4 all have enough homology to forma short RNA double helix ( hairpin loop )
When histidine is low, the ribosome stalls on top of region 1, and regions 2 and 3 can bind together. This is not a terminator, so histidine biosynthetic genes are made.
However, when there is a lot of histidine, the ribosome speeds over region 1 and gets into region 2. Thus, region 3 is free to bind to region 4. The region-3-region-4 hairpin loop is a terminator, so the RNA strand breaks and the histidine biosynthetic genes are not made.