A position effect in the control of transcription at yeast mating type loci

Abstract
The two mating type loci MATa and MATα each produce two mRNAs that are transcribed in opposite and diverging directions from central promoters. Silent copies of MATa (HMRa) and MATα (HMLα) contain identical DNA sequences throughout the transcribed region, yet are not transcribed. It is concluded that sequences to the left of HMRa (and probably HMLα) must somehow affect transcription initiated at the centre of each locus 700 to 1,400 base pairs away. A possible mechanism for this position effect is discussed.