HETEROGENEITY IN THE 5' UNTRANSLATED REGION OF MOUSE CYTOCHROME-C(T), MESSENGER-RNAS LEADS TO ALTERED TRANSLATIONAL STATUS OF THE MESSENGER-RNAS

Previous studies have shown that the differential regulation of mouse somatic cytochrome c (cyt C-S) and testicular cytochrome c (cyt C-T) during spermatogenesis is accompanied by changes in mRNA length [Hake et al. (1990) Development, 110, 249 - 257]. When analyzed by polysomal gradient sedimentation, cytochrome C-T sediment in two broad size classes: non-polysomal mRNAs are about 0.6 to 0.75 kb and polysomal mRNAs range from 0.7 to 0.9 kb. Both classes of mRNAs shorten to about 0.5 kb following deadenylation. Oligonucleotide-directed cleavage of the cytochrome C-T RNAs by RNase H reveals that the size heterogeneity of cytochrome C-T mRNAs resides in the 5' untranslated regions (UTRs). Ribonuclease protection assays reveal that multiple cytochrome C-T mRNAs are transcribed from six different transcriptional start sites spanning a region of 59 nucleotides in the 5'UTR from + 1 to + 59. Transcripts derived from the first;and second transcriptional initiation sites are not loaded onto polysomes as efficiently as those transcripts initiated from the other start sites. Each of the longer mRNAs has an upstream open reading frame, which starts at +8 and ends at +136 in the 5'UTR of the cytochrome C-T transcript. Computer analysis suggests that the lengthened 5' UTR sequences allow additional hairpin structures to be formed. Since the upstream open reading frame and the additional stem loop structure are absent in the 5' UTRs of the cytochrome C-T mRNAs initiated from the four downstream start sites, we suggest that these sequences in the two longest cytochrome C-T transcripts hinder their loading onto polysomes.