Describe the use of promoters in gene technology

Promoters switch genes on, if a gene is inserted into a bacterium it needs a promoter added to coordinate with the gene for the chanracteristic to be expressed.
For human insulin production in bacteria:
1) Gene intersted next to the B-galactosidase (lactase) gene so that they share a promoter.
2) Bacteria are grown in a medium containing lactose.
3) Lactase gene is switched on by promoter which switches on the insulin gene at the same time.
4) B-galactosidase and Human insulin are synthesised by the Bacteria, excreted and harvested for medical use and research.