The genome of archaeal prophage ΨM100 encodes the lytic enzyme responsible for autolysis of Methanothermobacter wolfeii

Methanothermobacter wolfeii (formerly Methanobacterium wolfei), a thermophilic methanoarchaeon whose cultures lyse upon hydrogen starvation, carries a defective prophage called Psi M100 on its chromosome. The nucleotide sequence of Psi M100 and its flanking regions was established and compared to that of the previously sequenced phage Psi M2 of Methanothermobacter marburgensis (formerly Methanobacterium thermoautotrophicum Marburg). The Psi M100 genome extends over 28,798 bp, and its borders are defined by flanking 21-bp direct repeats of a pure-AT sequence, which very likely forms the core of the putative attachment site where the crossing over occurred during integration. A large fragment of 2,793 bp, Wa, apparently inserted into Psi M100 but is absent in the genome of Psi M2. The remaining part of the Psi M100 genome showed 70.8% nucleotide sequence identity to the whole genome of Psi M2. Thirty-four open reading frames (ORFs) on the forward strand and one ORF on the reverse strand were identified in the Psi M100 genome. Comparison of Psi M100-encoded ORFs to those encoded by phage Psi M2 and to other known protein sequences permitted the assignment of putative functions to some ORFs. The ORF28 protein of Psi M100 was identified as the previously known autolytic enzyme pseudomurein endoisopeptidase PeiW produced by M. wolfeii.