Three types of Amy-2-related DNA sequences, Amy-2a I, Amy-2a II and Amy-X, exist in the genome of mice of the inbred strain A/J. Amy-2a I and Amy-X are single copy sequences. Amy-2a II occurs as three copies per haploid genome. DNA sequence analysis reveals that both classes of Amy-2a genes specify the same unique pancreatic alpha-amylase mRNA species, since they share common exon sequences. Four independently cloned Amy-2a II isolates were found to be identical in all regions sequenced. This suggests that most, if not all, chromosomal Amy-2a II copies are identical. Amy-X is presumably a pseudogene, since its exon sequences, which are distinct from those of Amy-2a, are not detected in pancreatic alpha-amylase mRNA. We have determined the transcriptional activities of the Amy-2a genes by mapping in vitro elongated nascent transcripts to Amy-2a restriction fragments. Transcription initiation occurs at or close to the cap site. The expression of Amy-2a in vivo is under control of strong promoters, which are active exclusively in the pancreas. The accumulation of alpha-amylase mRNA in cells of the exocrine pancreas is regulated mainly at the transcriptional level. We have searched for pancreatic transcripts of Amy-1a, which specifies both parotid gland and liver-type alpha-amylase mRNAs. Surprisingly, the weak Amy-1a promoter, which directs the synthesis of the mRNA containing the liver-type leader sequence, also is active in the pancreas and, hence, in all alpha-amylase-producing tissues.
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