Crystallin gene expression during rat lens development.

The analysis of the developmental pattern of the alpha A-, alpha B-, beta B1-, beta B2-, beta B3-, beta A3/A1-, and beta s-crystallin genes during fetal and postnatal development of the rat shows that the differential regulation of crystallin synthesis relies on differential gene shutdown rather than differential gene activation; that is, all crystallin genes are active during early development but turn off at different stages. The only two exceptions to this rule are the alpha B- and beta s-crystallin genes. The alpha B-crystallin gene transcript becomes first detectable at 18 days of fetal development, while the beta s-crystallin gene appears to be active only in the postnatal period. We also determined the absolute numbers of the alpha A-, alpha B-, beta B1-, beta B2-, beta B3-, beta A3/A1-, beta s-, and gamma-crystallin gene transcripts present in the lens at various times after birth. Comparison of these RNA data with the published protein data shows that the alpha B- and beta B2-crystallin RNAs are relatively overrepresented, suggesting the possibility that these two RNA species are not used as efficiently as other crystallin mRNAs. Examination of the known (hamster) alpha B-crystallin sequence and elucidation of the (rat) beta B2-crystallin sequence yielded no evidence for aberrant codon usage. These two RNAs have one sequence motif in common: they are the only crystallin mRNAs in which the translation initiation codon is preceded by CCACC.