IDENTIFICATION OF A 6TH PROMOTER THAT DIRECTS THE TRANSCRIPTION OF GAMMA-GLUTAMYL-TRANSPEPTIDASE TYPE-III RNA IN MOUSE

We have previously identified five promoters in the 5'-flanking region of the mouse gamma-glutamyl transpeptidase (gamma GT) gene. We now re port the localization of a sixth promoter that supports the transcript ion of type III RNA, the major gamma GT RNA in fetal liver. We made a fetal liver cDNA library enriched for gamma GT RNA and obtained 12 gam ma GT type III-specific clones. The longest clone is consistent with a transcription start site for type III RNA at a position 5' to the typ e IV promoter and about 5 kilobase(s) (kb) 5' to the first coding exon . We estimated by ribonuclease protection assay that about 80% of the gamma GT mRNA in fetal liver was type III. Primer extension and nuclea se protection analyses mapped the 5' end of type III mRNA in fetal liv er and kidney to a single cluster of potential major and minor transcr iption start sites. Deletion analysis using transient expression of ch loramphenicol acetyltransferase constructs of the type III promoter re gion revealed the greatest activity with a 1-kb 5'-flanking fragment i n mouse kidney proximal tubular cells and no detectable activity in NI H-3T3 fibroblasts. These studies demonstrate that the type III 5' regi on of the mouse gamma GT gene is organized into two distinct exons (II Ia and IIIb) and that type III RNA is expressed under the control of i ts own promoter.