Molecular cloning of glucokinase cDNA. Developmental and dietary regulation of glucokinase mRNA in rat liver.

A rat liver cDNA library enriched for glucokinase sequences was constructed using the phage expression vector lambda gt11 and screened with an antiserum to glucokinase. A positive phage clone termed lambda-GK223 was isolated by several rounds of plaque purification. When introduced in the high frequency lysogenization strain Y1089, the phage was shown to encode a fusion protein containing epitopes specific to rat liver glucokinase. The 1800-base pair cDNA insert of lambda-GK223 was subcloned in a pUC plasmid, and a resulting recombinant termed pUC-GK1 was used for hybrid selection of mRNA. The selected mRNA directed the synthesis in a cell-free translation system of a protein identified as glucokinase by electrophoresis and immunoprecipitation. The cloned cDNA was then used as a probe to measure the amount of glucokinase mRNA in rat liver during postnatal development. Glucokinase mRNA, 2.4 kilobases in length, was first detectable at day 14 after birth and increased 40-fold in amount from this age to day 31, in parallel with the emergence of glucokinase enzyme activity. In the adult rat, glucokinase mRNA was low during fasting and increased more than 50-fold above the fasting level within 6 h of an oral glucose load. However, maximal accumulation of glucokinase mRNA was short-lived and the mRNA level returned toward basal values by 18 h of refeeding. These data point to rapid and massive effects on the expression of the glucokinase gene at the transcriptional or post-transcriptional levels during ontogenic development and dietary changes in the adult animal.