The retinoblastoma orthologue, rblA, is a major regulator of S-phase, mitotic, and developmental gene expression in Dictyostelium

The retinoblastoma tumour suppressor, Rb, has two major functions. First, it represses genes whose products are required for S-phase entry and progression, thus stabilizing cells in G1. Second, Rb synergizes with factors that induce cell cycle exit and terminal differentiation. Dictyostelium lacks a G1 phase in its cell cycle but it has a retinoblastoma orthologue, rblA. Using mRNA-Seq transcriptional profiling, we show that rblA strongly represses hundreds of genes whose products are involved in S-phase and mitosis. Both S-phase and mitotic genes are expressed at a single point in late G2 and again in mid-development, near the time when cell cycling is reactivated. RblA also activates a set of genes unique to slime moulds that function in terminal differentiation. Like its mammalian counterpart, Dictyostelium RblA plays a dual role, regulating cell cycle progression and transcriptional events leading to terminal differentiation. In the absence of a G1 phase, however, RblA functions in late G2 controlling the expression of both S-phase and mitotic genes. Overall design: RNA was isolated from an rblA-disruptant and the parental strain when the cells were vegetatively growing or while the cells were developing (at the early culminant stage). Total RNA was also isolated from wild type cells synchronized using the cold-shock protocol (Maeda, J Gen Microbiol 132:1189-1196 (1986)). T0 to T13 represent the hours after the shift up from 9.5C to 22C, a regime that synchronizes the cells in the cell cycle.