The strategy of infection as a criterion for phylogenetic relationships of non-coli phages morphologically similar to phage T7.
Five phages which are morphologically similar to coliphage T7 but attack other host bacteria have been compared to T7 and to its relative, T3, by the following criteria: (a) cross-reactivity with antisera against T7 and T3, (b) DNA base sequence homologies, as determined by the C0t technique, (c) synthesis of two phage-coded enzymes: RNA polymerase and SAMase, (d) patterns of phage-directed protein synthesis, as determined by SDS-polyacrylamide gel electrophoresis of phage coat subunits. As judged by all these criteria, Pseudomonas phage PX3 is not related to T7; thus, morphological similarity was attributed to convergent evolution. The other phages, i.e. Serratia phage IV, Psuedomonas phage gh-1, Citrobacter phage ViIII and Klebsiella phage No. 11, were considered to be related to T7 on the basis of similarities in the patterns of phage-coded proteins and because, early after infection, these phages induced, as T7 does, an RNA polymerase which specifically transcribes the DNA of thehomologous phage. Phages IV and No. 11 also induced the early synthesis of SAMase (previously only known to occur upon T3 infection). With the exception of phage IV, however, DNA base sequence homologies with T7 or T3 seem to be poor or non-existent. The tested phages, again with the exception of phage IV, did not react with antiserum against T3 or T7. It is concluded that a particular pattern of phage-directed protein synthesis (as characterized by polyacrylamide gel electrophoresis and enzyme tests) may provide evidence for phylogenetic relationships between phages, even in cases where other criteria, such as genetic recombination, serological cross-reaction, and DNA base sequence homologies, fail to indicate relatedness.