Development of an in vitro bacteriophage N4 DNA replication system

Identification, cloning, and characterization of the bacteriophage N4 gene encoding the single-stranded DNA-binding protein. A protein required for phage replication, recombination, and late transcription

The coliphage N4-coded single-stranded DNA-binding protein (N4SSB) is essential for phage replication and for expression of the phage late genes, which are transcribed by the Escherichia coli sigma 70 RNA polymerase. As a first step in investigating the role of N4SSB in replication and transcriptional activation, we have identified and sequenced the N4SSB gene. The gene encodes a 265-amino acid protein with no apparent sequence homology to other single-stranded DNA-binding proteins. We present data indicating that N4SSB is also essential for phage recombination. Mutational analysis of the carboxyl terminus of the protein indicates that this region is required for protein-protein interactions with the N4 replication, N4 recombination, and E. coli transcriptional machineries, while the rest of the protein contains the determinants for single-stranded DNA binding.

Structure of the ends of the coliphage N4 genome

The coliphage N4 genome, a linear and double-stranded DNA of approximately 72,000 bases in length, has unique (non-permuted) direct terminal repeats of 390 to 440 base-pairs in length with 3' extensions. The very terminal sequences were determined by the Maxam-Gilbert method after 5' or 3' labeling, while sequences of internal fragments were determined by the dideoxy chain terminator method after cloning them onto M13 phage DNA. The left end of the N4 genome is relatively precise at its 5' terminus, while microheterogeneity of length exists at the 3'-terminal extensions. The predominant species had a 5 or 6 base 3' protruding sequence, 3' CATAA or 3' CATAAA. On the other hand, the right end is variable; there are at least six discrete ends differing from each other by approximately ten base-pairs and giving rise to the variability of the length of the terminal repeats. Each of the six discrete ends has a microheterogeneity of length, especially at the 3' termini. These properties of the terminal redundancy are discussed in conjunction with the mechanism whereby N4 DNA is replicated and processed.