A unique role of an amino terminal 16-residue region of long-type GATA-6.

Human GATA-6 mRNA utilizes two Met-codons in frame as translational initiation codons in cultured mammalian cells. An internal ribosome entry site (IRES) is not present in front of the coding region for short-type GATA-6. The 5'-upstream sequence with a short upstream open reading frame (uORF) did not affect the production of either long- or short-type GATA-6. Introduction of a canonical Kozak sequence around the upstream Met-codon resulted in predominant synthesis of long-type GATA-6, suggesting that the translation of short-type GATA-6 could be due to leaky scanning of the Met-codon by ribosomes. We found that at least the sequence comprising the 90th to 139th nucleotide bases from the first letter of the upstream Met-codon plays a positive role in the expression of long-type GATA-6. This was confirmed by insertion of the corresponding sequence in frame at the site of deletion (the 38th to 304th nucleotide residues). However, insertion of the sequence comprising the 92nd to 141st bases did not suppress the negative effect of the deletion. These results suggest that the translation of this region (Glu-31-Cys-46) could be critical for the apparent production of long-type GATA-6. We also demonstrated that long-type GATA-6 is potentially more active than the short-type.