HISTONE H1 OVEREXPRESSED TO HIGH-LEVEL IN TOBACCO AFFECTS CERTAIN DEVELOPMENTAL PROGRAMS BUT HAS LIMITED EFFECT ON BASAL CELLULAR FUNCTIONS

Histone H1, a major structural component of chromatin fiber, is believ ed to act as a general repressor of transcription. To investigate in v ivo the role of this protein in transcription regulation during develo pment of a multicellular organism, we made transgenic tobacco plants t hat overexpress the gene for Arabidopsis histone H1. In all plants tha t overexpressed H1 the total H1-to-DNA ratio in chromatin increased 2. 3-2.8 times compared with the physiological level. This was accompanie d by 50-100% decrease of native tobacco H1. The phenotypic changes in H1-overexpressing plants ranged from mild to severe perturbations in m orphological appearance and flowering. No correlation was observed bet ween the extent of phenotypic change and the variation in the amount o f overexpressed H1 or the presence or absence of the native tobacco H1 . However, the severe phenotypic changes were correlated with early oc currence during plant growth of cells with abnormally heterochromatini zed nuclei. Such cells occurred considerably later in plants with mild er changes. Surprisingly, the ability of cells with highly heterochrom atinized nuclei to fulfill basic physiological functions, including di fferentiation, was not markedly hampered. The results support the sugg estion that chromatin structural changes dependent on H1 stoichiometry and on the profile of major H1 variants have Limited regulatory effec t on the activity of genes that control basal cellular functions. Howe ver, the H1-mediated chromatin changes can be of much greater importan ce for the regulation of genes involved in control of specific develop mental programs.