The Size Heterogeneity of Rat Insulin-Like Growth Factor-I mRNAs is Due Primarily to Differences in the Length of 3′-Untranslated Sequence

Publisher Website
Google Scholar
Abstract
Previous studies have revealed multiple size classes of rat insulin-like growth factor-I (IGF-I) of estimated size 7.5-7.0, 1.9-1.5, and 1.2-0.9 kilobases (kb). Available sequence information accounts for only 2.1 kb of the 7.5-7.0 kb IGF-I mRNAs. We used oligomer directed ribonuclease H (RNase H) mapping to define the extent to which the unknown sequence in the large molecular weight mRNAs lies 5'' or 3'' to known sequence. Rat liver polyadenylated RNAs were incubated with oligomer probes complementary to internal rat IGF-I precursor (E domain) coding sequences. RNase H was used to hydrolyze IGF-I mRNAs at the point of annealment with the oligomers. Resultant 5'' and 3''-IGF-I mRNA fragments were analyzed on Northern blots. A probe specific for type 1 (class C) 5''-sequences (the most predominant of multiple 5''-sequence types found on rat IGF-I mRNAs) identifies interact IGF-I mRNAs of 7.5-7.0, 1.9-1.5 and 1.2-0.9 kb but, after oligomer directed RNase cleavage of these mRNAs, identified only a single IGF-I mRNA 5''-fragment. Major differences in the length of sequence 5'' to the IGF-I coding sequence therefore, do not account for the multiple size classes of type 1 (class C) IGF-I mRNAs. The size of the 5''-fragment suggests that the extent of sequence 5'' to the IGF-I coding sequence i s0.4-0.7 kb in type 1 (class C) IGF-I mRNAs. Identification of multiple 3''-fragments of IGF-I mRNAs demonstrated heterogeneity in the 3''-ends of rat IGF-I mRNAs. The sizes of the 3''-fragments indicate that the largest 7.5-7.0 kb IGF-I mRNAs have 3''-untranslated sequences (3''UT) of 6.7-6.2kb and that the smaller 1.9-1.5 and 1.2-0.9 kb IGF-I mRNAs have 3''UTs of 1.4-1.2 and 0.5-0.2 kb respectively. These differences in lenght of 3'' UT can account for the multiple size classes of rat liver IGF-I mRNAs and establish that the majority of unknown sequence in the large mol wt 7.5-7.0 kb IGF-I mRNAs resides in the 3''UT.