Identified S. cephaloptera operon within the BAC 35YA21. (a) Structure of the 158 kb BAC 35A21, including the predicted genes and the mapping of ESTs that bear SLs (purple). Detailed EST/BAC alignments are provided as Additional data files 15-17. (b) Detailed structure of the identified chætognath operon with RP S14 and PCNA genes and the corresponding ESTs (purple) that exhibit SL sequences. UTR (genomic, light orange; EST, light purple) and coding sequences (genomic, orange; EST, purple). (c) Alignments of selected regions of the operon, beginning and end of genes showing genomic DNA (orange) and transcripts with their alternative SL forms. The very short distance encountered between the end and beginning of the two genes argues for polycistronic transcription.