Analysis of six prophages in Lactococcus lactis IL1403: different genetic structure of temperate and virulent phage populations

We report the genetic organisation of six prophages present in the genome o f Lactococcus lactis IL1403, The three larger prophages (36-42 kb), belong to the already described P335 group of temperate phages, whereas the three smaller ones (13-15 kb) are most probably satellites relying on helper phag e(s) for multiplication, These data give a new insight into the genetic str ucture of lactococcal phage populations, P335 temperate phages have variabl e genomes, sharing homology over only 10-33% of their length. In contrast, virulent phages have highly similar genomes sharing homology over >90% of t heir length, Further analysis of genetic structure in all known groups of p hages active on other bacterial hosts such as Escherichia coli, Bacillus su btilis, Mycobacterium and Streptococcus thermophilus confirmed the existenc e of two types of genetic structure related to the phage way of life. This might reflect different intensities of horizontal DNA exchange: low among p urely virulent phages and high among temperate phages and their lytic homol ogues, We suggest that the constraints on genetic exchange among purely vir ulent phages reflect their optimal genetic organisation, adapted to a more specialised and extreme form of parasitism than temperate/lytic phages.