DIFFERENTIAL EFFECT OF H1 VARIANT OVEREXPRESSION ON CELL-CYCLE PROGRESSION AND GENE-EXPRESSION

To identify functional differences among non-allelic variants of the m ammalian H1 linker histones a system for the overexpression of individ ual H1 variants in vivo was developed, Mouse 3T3 cells were transforme d with an expression vector containing the coding regions for the H1c or H1(0) variant under the control of an inducible promoter, Stable, s ingle colony transformants, in which the normal stoichiometry of H1 va riants was perturbed, displayed normal viability, unaltered morphology and no long-term growth arrest, However, upon release from synchroniz ation at different points in the cell cycle transformants significantl y overproducing H1(0) exhibited transient inhibition of both G(1) and S phase progression, Overexpression of H1c to comparable levels had no effect on cell cycle progression, Analysis of transcript levels for s everal cell cycle-regulated and housekeeping genes indicated that over expression of H1(0) resulted in significantly reduced expression of al l genes tested, Surprisingly, overexpression of H1c to comparable leve ls resulted in either a negligible effect or, in some cases, a dramati c increase in transcript levels, These results support the suggestion that functional differences exist among H1 variants.