Transcription-coupled translation control of AML1/RUNX1 is mediated by cap- and internal ribosome entry site-dependent mechanisms

AML1/RUNX1 belongs to the runt domain transcription factors that are import ant regulators of hematopoiesis and osteogenesis. Expression of AML1 is reg ulated at the level of transcription by two promoters, distal (D) and proxi mal (P), that give rise to mRNAs bearing two distinct 5' untranslated regio ns (5'UTRs) (D-UTR and P-UTR). Here we show that these 5'UTRs act as transl ation regulators in vivo. AML1 mRNAs bearing the uncommonly long (1,631-bp) P-UTR are poorly translated, whereas those with the shorter (452-bp) D-UTR are readily translated. The low translational efficiency of the P-UTR is a ttributed to its length and the cis-acting elements along it. Transfections and in vitro assays with bicistronic constructs demonstrate that the D-UTR mediates cap-dependent translation whereas the P-UTR mediates cap-independ ent translation and contains a functional internal ribosome entry site (IRE S). The IRES-containing bicistronic constructs are more active in hematopoi etic cell lines that normally express the P-UTR-containing mRNAs. Furthermo re, we show that the IRES-dependent translation increases during megakaryoc ytic differentiation but not during erythroid differentiation, of K562 cell s. These results strongly suggest that the function of the P-UTR IRES-depen dent translation in vivo is to tightly regulate the translation of AML1 mRN As. The data show that AML1 expression is regulated through usage of altern ative promoters coupled with IRES-mediated translation control. This IRES-m ediated translation regulation adds an important new dimension to the fine- tuned control of AML1 expression.