Tissue-specific heterogeneity of the 3'-untranslated region of L-type pyruvate kinase mRNAs

Abstract

A single L-type pyruvate kinase (PK) gene seems to exist per haploid genome. It is expressed in the liver, kidney and small intestine in the form of three mRNA species of 2, 2.2 and 3.2 X 10(3) bases (kb). All three species are polyadenylated and translatable into the same L-type subunit. Primer extension experiments demonstrate that all three PK mRNAs have the same 5' ends. Nuclease S1 protection experiments with various cDNA and 3' genomic probes indicate that the different mRNA species only differ by the length of their 3' noncoding region. The mechanism responsible for the production of the three transcripts seems to be the use of alternative unusual polyadenylation sites. Run-on assays with specific probes recognizing only the 3.2-kb or all three mRNA species show that the transcription proceeds across the gene with similar rate. This means that the process involved in generation of the three transcripts is a posttranscriptional event, probably due to different sites of endonucleolytic cleavage of primary transcripts extending 3' from the gene region encoding the mature mRNAs. The ratio between the different PK mRNA species is, to a certain extent, tissue-specific and changes with development. The role of an 'identifier sequence' located in the 3' noncoding sequence of the 3.2-kb species in such a tissue-specific use of alternative polyadenylation sites is discussed.