Specialization and targeting of B-type cyclins

The B-type cyclins of S. cerevisiae are diversified with respect to time of expression during the cell cycle as well as biological function. We replac ed the early-expressed CLB5 coding sequence with the late-expressed CLB2 co ding sequence, at the CLB5 locus. CLB5::CLB2 exhibited almost no rescue of clb5-specific replication defects, although it could rescue clb1 clb2 letha lity, and in synchronized cells Clb2p-associated kinase activity from CLB5: :CLB2 rose early in the cell cycle, similar to that of Clb5p. Mutagenesis o f a potential substrate-targeting domain of CLB5 reduced biological activit y without reducing Clb5p-associated kinase activity. Thus, Clb5p may have t argeting domains required for CLB5-specific biological activity.