Expression of p8 during the cell cycle. (A) Northern blot analysis of total
RNA isolated from growing myoblasts (Mb) and arrested myoblasts (G0) of the p8
gene-trap clone gtQ39 (top panels) and parental C2C12 cells (bottom panels).
In gtQ39 cells, the β-galactosidase (βgal) probe detects a gene-trap
fusion transcript (exon 1 of p8 fused to the βgal-coding sequences) that
is upregulated ∼fivefold in G0. Expression of endogenous p8 RNA
in parental C2C12 cells (detected using a p8-specific probe) is also induced
>fivefold by arrest. 28S rRNA and L7 RNA serve as loading controls. (B) p8
expression is further activated during the G0-G1 transition, reaching 20-fold
induction in mid-G1. G0, northern blot analysis of arrested myoblasts 48 hours
after induction of arrest; R2-24, G0-synchronized cells reactivated into the
cell cycle for the indicated number of hours. Histone mRNA reports the
proportion of cells in S phase. (C) MyoD expression during a timecourse of
reversible arrest as in B. Expression of the MyoD transcript is
induced in G1, later than p8 expression.