We have compared the appearance in the small intestine of baby rabbits, of sucrase and isomaltase activities and of the sucrase-isomaltase mRNA. For the latter we used a cDNA probe encompassing ~4.1 kb from the 5'-end of pro-sucrase-isomaltase cDNA [(1986) Cell 46, 227-234]. Sucrase-isomaltase mRNA and the enzyme activities developed simultaneously from the 15th day after birth onwards. Over two orders of magnitude the enzymatic activities and sucrase-isomaltase mRNA matched one another closely, thus ruling out translation as the main site of biosynthetic control during spontaneous development, while rendering very probable transcription as the primary site of control. However, we cannot rule out the possibility that, prior to day 15, sucrase-isomaltase mRNA might be degraded so rapidly that it is not translated.
