Organization of 5′ region of b5R gene (A) and predicted translation products of rat and human transcripts (B). (A) In rats and humans alternative promoters generate two transcripts differing in the first exon (1M or 1S). (B) M and S show the 5′ portions of b5R M- and S-transcripts in humans and rat and the corresponding deduced amino acid sequences. The myristoylation consensus is underlined and the 14 uncharged residues, which complete the membrane anchor, are overlined and underlined. The heavy vertical bars indicate the junction between first and second exon. The light vertical lines show positions of amino acid identity between man and rat. Translation of the rat M transcript initiates exclusively from the first AUG contained in the 1M exon (heavy arrow), so that the internal AUG of exon 2 is not used (dashed arrow).10 Also the human 1M exon contains an AUG in an optimal context for initiation, however the possible use of the two downstream AUG codons has not been directly tested. The rat 1S exon contains an in-frame AUG in a poor context for initiation, which is used inefficiently (dashed arrow), so that the main translation product is from the downstream AUG (heavy arrow), with generation of soluble b5R.10 The human 1S exon is devoid of AUG codons, however, exon 2 contains an additional AUG at the beginning of the sequence coding for the hydrophobic stretch, which could in principle be used as initiation codon.