Mutations in the terminase genes of bacteriophage λ that bypass the necessity for FI

DNA maturation in bacteriophage λ is the process by which the concatemeric precursor DNA is cleaved at sites called cos to generate mature λ DNA molecules. These DNA molecules are then packaged into procapsids, the empty capsid precursors. The enzyme that catalyses these events is λ DNA terminase. It is composed of two subunits, made of 181 and 641 amino acids, the products of genes Nu1 and A, respectively. The product of the FI gene (gp FI  ) stimulates the formation of an intermediate in capsid assembly called complex II, which contains a procapsid, terminase and DNA. The mechanism of stimulation remains unknown. It has been suggested that gp FI may also stimulate terminase‐mediated cos cleavage, in the absence of procapsids, by increasing enzyme turnover. Mutants in FI fail to mature and package DNA but, in comparison with other capsid gene mutants, FI mutants are leaky. Second site mutants of FI phages, called ‘fin’ (for FI independence), bypass the necessity for gp FI . These mutants were originally localized to the region of Nu1 and A and are of two classes: finA includes those that induce the synthesis of fourfold more gene A product (gp A  ) than wild‐type phages, and finB includes those that produce normal amounts of gp A . Whereas all finA mutants analysed map to Nu1 , finB mutants have been found both in E and in Nu1. The existence of E mutants able to bypass the necessity for gp FI in vivo shows that gp E and gp FI interact, directly or indirectly. Here we have analysed and sequenced two finA mutants and one finB mutant. All of these map in Nu1 . Of the two finA mutants, one corresponds to an Ala163Ser change and the other is a silent mutation. It is likely that the finA mutations alter mRNA conformation in a manner that results in an increase in the efficiency of A mRNA translation. The fourfold increase in gp A synthesis translates into a 10‐fold increase in terminase activity. These results show that terminase overproduction is sufficient to bypass the necessity for gp FI and that such an overproduction can be achieved by changes in the efficiency of translation of A due to subtle changes in the sequence upstream of the gene. The finB cs 103 mutation is a His‐87→Tyr change in Nu1 . Therefore, an alternative way in which to bypass the requirement for gp FI involves an alteration in the structure of gp Nu1 . It is likely that the altered gp Nu1 would increase cleavage and packaging efficiency directly or indirectly. We have determined that DNA cleavage in vivo does not occur in the absence of gp FI . Therefore it seems that gp FI somehow facilitates an otherwise latent capacity of terminase to autoactivate its nucleolytic activity.