An alternate promoter in the glucokinase gene is active in the pancreatic beta cell.

An alternate promoter in the glucokinase gene is active in the beta cell and produces a glucokinase mRNA which is longer and that has a different leader sequence and translation start site than the hepatic glucokinase mRNA. The glucokinase beta cell promoter is located at least 12 kilobases upstream from the glucokinase hepatic promoter. Transcription from the glucokinase beta cell promoter initiates over a region of 62 bases. The absence of a TATA box homology in the proximal promoter region may account for the diffuse transcriptional initiation. Translation of the beta cell glucokinase mRNA predicts a glucokinase isozyme that is different from the hepatic isozyme by 15 amino acids at the N terminus. The use of alternative promoters apparently enables the glucokinase gene to be regulated by insulin in the liver and by glucose in the beta cell, thus possibly constituting an important feedback control loop for maintaining glucose homeostasis. Alternate RNA splicing of the beta cell glucokinase mRNA predicts at least two beta cell glucokinase isoforms. An alternate splice acceptor site in the 4th exon of the glucokinase gene was identified in two glucokinase cDNAs from rat insulinoma tissue. Use of the alternate splice acceptor site results in a 51-nucleotide in frame deletion in the beta cell glucokinase mRNA and removal of 17 amino acids from a region of the protein situated between the putative glucose and ATP binding domains. Analysis of the pattern of RNA splicing in tissues containing beta cells indicates that the splice acceptor site utilized in producing hepatic glucokinase mRNA is also utilized in the beta cell.