Crystallin gene expression during rat lens development

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Abstract
The analysis of the developmental pattern of the .alpha.A-, .alpha.B-, .beta.B1-, .beta.B2-, .beta.B3-, .beta.A3A1-, 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.-crystalline 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.