Gene and locus are the same, rather synonyms. In fact, the protein from the gene is translated by mRNA that is transcribed by the gene/locus.
Promoter is the part of gene/locus from where the transcription begins for mRNA transcript to be transcribed from the locus.
A gene or locus should have promoter, operator and intact protein coding region for a transcript to be transcribed accurately with no errors; when that happens and the translation machinery in the cell is intact then the protein is translated from mRNA of the gene/locus.
Hope this helps.