THE ASIA GENE-PRODUCT OF BACTERIOPHAGE-T4 IS REQUIRED FOR MIDDLE MODERNA-SYNTHESIS

The asiA gene of bacteriophage T4 encodes a 10-kDa peptide which binds strongly in vitro to the sigma(70) subunit of Escherichia coli RNA po lymerase, thereby weakening sigma(70)-core interactions and inhibiting sigma(70)-dependent transcription. To assess the physiological role o f this protein, we have introduced an amber mutation into the proximal portion of the asiA gene. On suppressor-deficient hosts, this mutant phage (amS22) produces minute plaques and exhibits a pronounced delay in phage production. During these mutant infections, T4 DNA synthesis is strongly delayed, suggesting that the AsiA protein plays an importa nt role during the prereplicative period of phage T4 development. The kinetics of protein synthesis show clearly that while T4 early protein s are synthesized normally, those expressed primarily via the middle m ode exhibit a marked inhibition. In fact, the pattern of protein synth esis after amS22 infection resembles greatly that seen after infection by amG1, an amber mutant in motA, a T4 gene whose product is known to control middle mode RNA synthesis. The amber mutations in the motA an d asiA genes complement, both for phage growth and for normal kinetics of middle mode protein synthesis. Furthermore, primer extension analy ses show that three different MotA-dependent T4 middle promoters are n ot recognized after infection by the asiA mutant phage. Thus, in conju nction with the MotA protein, the AsiA protein is required for transcr iption activation at T4 middle mode promoters.