We have shown that changes in proportions of the four chicken H-1's during in vitro myogenesis are primarily the result of differential coupling of their synthesis to DNA replication (see the previous paper). We show here that the four major chicken H-1's are encoded by distinct mRNAs which specify primary amino acid sequence variants. Accumulation of the H-1-variant mRNAs is coupled to DNA replication to different extents. The level of mRNA encoding H-1c (the H-1 variant that increases relative to the other H-1's in nondividing muscle cells) is completely uncoupled. In contrast, the level of mRNAs encoding H-1's a, b, and d (which have levels that decrease in nondividing muscle cells) are more tightly coupled. Polyadenylation is not involved in uncoupling H-1c mRNA accumulation from DNA replication.

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