Linker histone H1 regulates specific gene expression but not global transcription in vivo.

In a linker histone H1 knockout strain (delta H1) of Tetrahymena thermophila, the number of mature RNAs produced by genes transcribed by pol I and pol III and of most genes transcribed by pol II remains unchanged. However, H1 is required for the normal basal repression of a gene (ngoA) in growing cells but is not required for its activated expression in starved cells. Surprisingly, H1 is required for the activated expression of another gene (CyP) in starved cells but not for its repression in growing cells. Thus, H1 does not have a major effect on global transcription but can act as either a positive or negative gene-specific regulator of transcription in vivo.