SummaryA118 is a temperate phage isolated from Listeria monocytogenes. In this study, we report the entire nucleotide sequence and structural analysis of its 40 834 bp DNA. Electron microscopic and enzymatic analyses revealed that the A118 genome is a linear, circularly permuted, terminally redundant collection of double-stranded DNA molecules. No evidence for cohesive ends or for a terminase recognition (pac) site could be obtained, suggesting that A118 viral DNA is packaged via a headful mechanism. Partial denaturation mapping of DNA cross-linked to the tail shaft indicated that DNA packaging proceeds from left to right with respect to the arbitrary genomic map and the direction of genes necessary for lytic development. Seventy-two open reading frames (ORFs) were identi®ed on the A118 genome, which are apparently organized in a life cycle-speci®c manner into at least three major transcriptional units. N-terminal amino acid sequencing, bioinformatic analyses and functional characterizations enabled the assignment of possible functions to 26 ORFs, which included DNA packaging proteins, morphopoetic proteins, lysis components, lysogeny control-associated functions and proteins necessary for DNA recombination, modi®ca-tion and replication. Comparative analysis of the A118 genome structure with other bacteriophages revealed local, but sometimes extensive, similarities to a number of phages spanning a broader phylogenetic range of various low GC host bacteria, which implies relatively recent exchange of genes or genetic modules. We have also identi®ed the A118 attachment site attP and the corresponding attB in Listeria monocytogenes, and show that site-speci®c integration of the A118 prophage by the A118 integrase occurs into a host gene homologous to comK of Bacillus subtilis, an autoregulatory gene specifying the major competence transcription factor.

KaiC from Synechococcus elongatus PCC 7942 (KaiC) is an essential circadian clock protein in cyanobacteria. Previous sequence analyses suggested its inclusion in the RecA͞DnaB superfamily. A characteristic of the proteins of this superfamily is that they form homohexameric complexes that bind DNA. We show here that KaiC also forms ring complexes with a central pore that can be visualized by electron microscopy. A combination of analytical ultracentrifugation and chromatographic analyses demonstrates that these complexes are hexameric. The association of KaiC molecules into hexamers depends on the presence of ATP. The KaiC sequence does not include the obvious DNA-binding motifs found in RecA or DnaB. Nevertheless, KaiC binds forked DNA substrates. These data support the inclusion of KaiC into the RecA͞DnaB superfamily and have important implications for enzymatic activity of KaiC in the circadian clock mechanism that regulates global changes in gene expression patterns.DnaB ͉ RecA ͉ cyanobacteria ͉ Synechococcus