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RNA sequences coding for the most abundant chicken lens proteins, delta-crystallin, were detected at very low levels in day old post hatch chick lung, heart, kidney and liver, and in 6 day embryo headless bodies. The pattern of cytosine methylation within the CCGG sequences of the delta-crystallin genes was also examined and shown to vary in several non-lens tissues, from several stages of development. Embryonic neural retina, which expresses a higher level of delta-crystallin RNA than the above tissues, is no less methylated in the sites studied than the tissues which have no association with the eye, and is actually more heavily methylated than the kidney. Thus no obvious correlation was found between undermethylation and gene expression.

We selected 24 clones from a λ charon 4A chicken genomic library (a gift from R.Axel) screened with cDNA made from day old chick lens total RNA. The 15 most strongly hybridizing clones carried δ- crystallin sequences overlapping each other to cover the whole 20kb region of the two tandemly arranged δ-crystallin genes.

The main protein of the pre-hatching chick is -crystallin. There are two -crystallin genes, (Bhat and Piatigorsky, 1979) and their sequences are similar enough to cross-hybridise under the most stringent conditions.

During development of the vertebrate lens, the lens epithelium undergoes a final stage of differentiation into lens fibre cells. Lens fibre cells can also be produced by trans-differentiation from certain extralenticular structures, all of which are of different developmental origin from lens, including embryonic neural retina and retinal pigmented epithelium. Delta-crystallin is the major lens protein in the chick and appears first in development; it is the major product in trans-differentiated retina of younger embryos. In both normal differentiation and trans-differentiation an increase of delta-crystallin coding RNA is detectable in the nucleus of cells prior to their terminal differentiation into lens fibres. The increase in transcription of delta-crystallin genes accompanying final differentiation of lens fibres, appears to take place slightly in advance of an increase in the capacity to process and transport this mRNA to the cytoplasm.