1.9kb and 2.1kb fragments of pb transcription unit covering exon 1, in normal orientation, governing expression of lacZ.
1.9kb NcoI-BamHI and 2.1kb BamHI-SalI fragment (intron 1 sequences and the conserved second intron sequence) of pb inserted upstream and in same orientation to lacZ.
Strong pairing sensitivity conferred by fused 1.9kb and 2.1kb pb fragment.