Liver-specific RNA processing of the ubiquitously transcribed rat
fibrinogen gamma-chain gene
PJ Haidaris and MA Courtney
Department of Medicine (Hematology Unit), University of Rochester School of
Medicine and Dentistry, NY.
Fibrinogen gamma-chains differ in amino acid sequence at the
carboxyterminus due to alternative 3' RNA processing. Previous studies
reported differences between humans and rats in the mechanism of gamma-
chain RNA processing and that it was a nonregulated event. To test the
hypothesis that rat gamma-chain RNA processing involves both alternative
splicing and poly(A) site selection and that it is regulated in a
tissue-specific manner, we determined the tissue distribution of
gamma-chain mRNA expression and the pattern of gamma- chain pre-mRNA
processing. The results of in situ hybridization demonstrated that gamma A
and gamma B transcripts were localized to and codistributed in liver
hepatocytes, indicating that no subset of cells process gamma B mRNA. The
ubiquitous expression of the fibrinogen gamma- chain promoter was
demonstrated in marrow, lung, brain, and liver by RNase protection using a
5'-specific gamma-chain probe. RNase protection studies to map 3' RNA
processing sites suggested that, in addition to the distal poly(A) signal
previously identified, two alternative poly(A) signals within the last
intron (ATTAAA and AATAAA) were used only in liver to produce gamma B
transcripts. Approximately equal usage of the three poly(A) signals (27%,
37%, and 36%, respectively) to form the 3' end of mature gamma B
transcripts suggested that poly(A) site selection is random. These results
indicate that splicing of the last intron to produce gamma A mRNA is the
ubiquitous and constitutive pattern of gamma-chain RNA processing, while
retention of the last intron to produce gamma B mRNAs is the
tissue-specific and regulated pattern of gamma-chain RNA processing. The
pattern of rat gamma-chain RNA processing is similar to human, implying
that the mechanism is conserved. These data support a mechanism of
tissue-specific splice site selection predominating over poly(A) site
selection in gamma-chain pre-mRNA processing. The expression of both
fibrinogen gamma-chain transcripts in liver, rather than mutually exclusive
expression in liver and other tissues, provides a new model for studying
tissue-specific alternative 3' end formation regulatory mechanisms.
Volume 79,
Issue 5,
pp. 1218-1224,
03/01/1992
Copyright © 1992 by The American Society of Hematology
