The first phase, which encompasses the first 4-6 hours following a blood meal, is characterized by the presence of small amounts of early trypsin. The second phase, which occurs between 8 and 36 hours after blood feeding, is characterized by the presence of large amounts of late trypsin. Transcription of the early trypsin gene starts a few hours after adult emergence and is under control of juvenile hormone. However, the early trypsin mRNA is stored in the midgut epithelium and remains untranslated until a blood meal is taken. The enzymatic activity of early trypsin plays a unigue and critical role in the regulation of late trypsin synthesis.
Early trypsin acts like a sensor to determine whether protein has entered the midgut.
It carries out limited proteolysis of the ingested proteins and, somehow, the products of this limited proteolysis induce transcription of the late trypsin gene. Late trypsin is the protease responsible for the majority of the endoproteolytic cleavage of the meal proteins. Once transcription has been activated, the extent of transcription of the late trypsin gene is proportional to the amount of protein present in the meal. In addition, the amount of late trypsin protein translation is controlled by the amount of amino acid released during digestion of the meal proteins. Regulation at both transcriptional and translational levels allows the midgut to adjust the amount of late trypsin with remarkable flexibility in response to a particular meal.