An essential GTPase which binds GTP, GDP and ppGpp with moderate affinity (with a twofold preference for GDP over GTP), shows high guanine nucleotide exchange rate constants for both nucleotides, and has a relatively low GTP hydrolysis rate. Deletion of the 159 N-terminal residues makes a protein that is non-functional in vivo but which retains nucleotide binding and GTPase activity. Required for cell cycle progression from G1 to S phase and for DNA replication. Monomer. Cosediments in sucrose gradients with the 50S ribosomal subunit. Constitutively expressed (at protein level). Essential for growth, it cannot be disrupted. As protein levels decrease translating ribosomes, but not individual subunits, also decrease. Site-specific mutations that are activating (168-Val) or dominant negative (173-Asn or 280-Tyr) in ras-like proteins do not have the same phenotype in this protein family. Belongs to the TRAFAC class OBG-HflX-like GTPase superfamily. OBG GTPase family.