Sentence:  Similar considerations apply in the case of tRNA genes, where the internal promoter is split into two functional domains (<WSD>box</WSD> A and <WSD>box</WSD> B) which must be a minimum distance apartnb.... The first 11 bp of the internal control region in the "Xenopus" 5S gene are structurally and functionally homologous to the <WSD>box</WSD> A element of tRNA gene promoters, ... 
Meaning of the word marked with <WSD> tags:  One of two specific regions in a promoter.