MECHANISMS OF GENOME PROPAGATION AND HELPER EXPLOITATION BY SATELLITEPHAGE-P4

Temperate coliphage P2 and satellite phage P4 have icosahedral capsids and contractile tails with side tail fibers. Because P4 requires all the capsid, tail, and lysis genes (late genes) of P2, the genomes of t hese phages are in constant communication during P4 development. The P 4 genome (11, 624 bp) and the P2 genome (33.8 kb) share homologous cos sites of 55 bp which are essential for generating 19-bp cohesive ends but are otherwise dissimilar. P4 turns on the expression of helper ph age late genes by two mechanisms: derepression of P2 prophage and tran sactivation of P2 late-gene promoters. P4 also exploits the morphopoie tic pathway of P2 by controlling the capsid size to fit its smaller ge nome. The P4 sid gene product is responsible for capsid size determina tion, and the P2 capsid gene product, gpN, is used to build both sizes . The P2 capsid contains 420 capsid protein subunits, and P4 contains 240 subunits. The size reduction appears to involve a major change of the whole hexamer complex. The P4 particles are less stable to heat in activation, unless their capsids are coated with a P4-encoded decorati on protein (the psu gene product). P4 uses a small RNA molecule as its immunity factor. Expression of P4 replication functions is prevented by premature transcription termination effected by this small RNA mole cule, which contains a sequence that is complementary to a sequence in the transcript that it terminates.