Investigations into size heterogeneity of the alpha B-crystallin mRNA

alpha B-crystallin is expressed in a variety of developmental, physiological and pathological conditions in a number of tissues. Based on northern blots of total RNA, the existence of at least two size classes of alpha B-crystallin mRNAs has been reported, one smaller predominant species, 0.8-0.9 kb, and the other 1.2 to 1.4 kb. This heterogeneity has been attributed to alternative upstream transcriptional initiation. We have investigated the origin of the size heterogeneity of alpha B-crystallin mRNA by using 5'-upstream-, coding- and 3'-untranslated-region probes in RNAse protection and northern blot assays. RNAse protection assays indicate that there is only one predominant initiation site as previously reported and that the second polyadenylation signal is not used in the rat gene. Importantly, northern blot data obtained with coding region-only probe shows that the size of alpha B mRNA detected in the heart and the lens is similar (0.78 kb) in poly A+ as well as in total RNA. On the other hand, in the brain and in the lung, the larger hybridizing species (1.05 kb and 1.22 kb respectively) seen in total RNA are not detected in poly A+RNA which shows a 0.95 kb species in both tissues. The 5' upstream probe (-1 to -499) produces weak hybridization patterns in the brain and the lung, similar to those obtained with coding region-only probe. The 5' probe did not show hybridization in the heart and the lens RNAs. These data suggest that upstream initiations represent a minor population of transcripts and that higher size transcripts (about 500 bp larger) actually represent non-polyadenylated RNAs that may not contribute to the generation of the actual gene product.