the present invention provides expression vectors useful for high - throughput screening of gene libraries . in a specific embodiment , an expression vector comprising the rop gene operatively linked to the trp promoter - operator ; a purification tag sequence and a protease cleavage site downstream of the rop gene ; and a multiple cloning site downstream of the protease cleavage site , wherein the insertion of a heterologous gene of interest into the multiple cloning site and subsequent expression thereof in a host cell produces a high yield of a fusion protein comprising the rop protein and the protein encoded by the heterologous gene of interest without the need of chemical inducers , temperature shifts , or growth medium alterations to initiate protein synthesis , and wherein the fusion protein controls plasmid replication at temperatures below about 30 ° c . but exhibits runaway plasmid replication when cultured at about 37 ° c .