The white halo plaque phenotype of bacteriophage T4: Its uses and applications in screening and mapping of splicing-defective mutants

Bacteriophage T4 is a remarkable organism that has played a vital role in the progress of molecular biology. The host-virus relationship of T4 and E. coli have been studied so well that biochemical and genetic manipulations can be conducted and detected with utmost precision. E. coli OK305, a derivative of E. coli B contains a defect in its pyrimidine metabolism, a very low cytidine deaminase activity and no detectable deoxycytidine deaminase activity. This makes OK305 the ideal host for studies using the white halo plaque phenotype. This phenomenon has been used extensively for screening of nrdB and td intron mutants that are splicing defective. This approach has further been extended for screening for revertants of these splicing defective mutants. Secondary structure models of nrdB and td introns have been postulated using this simple yet powerful approach. In this review, we discuss the white halo plaque phenotype, its potential usage and applications in screening and mapping of mutations and experimental designs used for isolation of intragenic and extragenic suppressors of primary splicing defective mutations.