Overproduction of an antisense RNA containing the oop RNA sequence of bacteriophage λ induces clear plaque formation

We have constructed an IPTG-inducible plasmid which overexpresses oop RNA sequences in Escherichia coli. Infection of these transformed E. coli cells (SB221/pOOP5) with λ⁺ phage produced clear plaques, whereas λ⁺ infection of cells transformed with the plasmid vector (SB221/pJDC406) or the plasmid expressing the oop RNA transcript in the other orientation (SB221/pOOP9) gave rise to turbid plaques characteristic of λ⁺. Calculations of the percentage of infected cells forming lysogens show a 6-fold decrease in the absence of isopropyl β-d-thiogalactoside (IPTG) and a 20-fold decrease in the presence of IPTG for SB221/pOOP5 as compared to both SB221/pJDC406 and SB221/pOOP9. We have thus shown that the overexpression of oop RNA favors the lytic mode of λ development.