This unit describes the maintenance and care of insect cell cultures as well as the generation, purification, and storage of recombinant baculoviruses. Procedures are included for maintenance and subculturing of insect cells and cotransfection of insect cells with linearized baculovirus DNA and recombinant transfer plasmid containing the gene of interest. In the event that the linearized virus is not available, wild-type baculovirus (AcMNPV) DNA may be used to produce recombinant baculoviruses. A procedure is also included for the generation of recombinant baculoviruses using a novel method, direct cloning, which eliminates the need to first clone the gene of interest into a baculoviral transfer vector. Preparation of baculovirus infection stocks from both monolayer and suspension cultures is also described. Finally, a protocol is given for a plaque assay to be used for determining the titer of baculoviral stocks as well as for selection of recombinants and plaque purification.
Studies have demonstrated that the retinoblastoma susceptibility gene product, RB, can either positively or negatively regulate expression of several genes through cis-acting elements in a cell-type-dependent manner. The nucleotide sequence of the retinoblastoma control element (RCE) motif, GCCACC or CCACCC, and the Sp1 consensus binding sequence, CCGCCC, can confer equal responsiveness to RB. Here, we report that RB activates transcription of the c-jun gene through the Sp1-binding site within the c-jun promoter. Preincubation of crude nuclear extracts with monoclonal antibodies to RB results in reduction of Sp1 complexes in a mobility shift assay, while addition of recombinant RB in mobility shift assay mixtures with CCL64 cell extracts leads to an enhancement of DNA-binding activity of SP1. These results suggest that RB is directly or indirectly involved in Sp1-DNA binding activity. A mechanism by which RB regulates transactivation is indicated by our detection of a heat-labile and protease-sensitive Sp1 negative regulator(s) (Sp1-I) that specifically inhibits Sp1 binding to a c-jun Sp1 site. This inhibition is reversed by addition of recombinant RB proteins, suggesting that RB stimulates Sp1-mediated transactivation by liberating Sp1 from Sp1-I. Additional evidence for Sp1-I involvement in Sp1-mediated transactivation was demonstrated by cotransfection of RB, GAL4-Sp1, and a GAL4-responsive template into CV-1 cells. Finally, we have identified Sp1-I, a approximately 20-kDa protein(s) that inhibits the Sp1 complexes from binding to DNA and that is also an RB-associated protein. These findings provide evidence for a functional link between two distinct classes of oncoproteins, RB and c-Jun, that are involved in the control of cell growth, and also define a novel mechanism for the regulation of c-jun expression.
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