Middle Transcription

In contrast to early (IE) genes, expression of middle (and late) T4 genes requires phage-directed protein synthesis for several reasons. Transcription from middle promoters requires prior synthesis of two early T4 proteins: AsiA and MotA. Alternatively, downstream genes of transcription units initiated from early promoters may require antitermination or stabilization of rare long transcripts (383-385). The distinction between different classes is blurred, however, because many T4 genes are under dual or multiple transcriptional controls, and transcription of middle (DE) genes may be delayed for several reasons. Moreover, many late T4 genes are transcribed from late promoters but are also cotranscribed with early genes from early or middle promoters. For these late genes, post-transcriptional mechanisms (discussed below) prevent expression at early times.

The AsiA protein (77, 166-168, 382) inhibits host transcription, because it binds to region 4 of ct70 (265, 316, 355, 356) and prevents transcription of host promoters with "standard" —35 regions. It has been postulated that AsiA also turns off early T4 promoters. However, experiments using asiA deletions indicate that other, yet unknown factors are important for the shutoff of most early T4 promoters (325), which have nonstandard —35 regions.

AsiA is also an activator of T4 middle promoters (166). At middle promoters the ct70 subunit, complexed with AsiA protein and core RNA polymerase, recognizes consensus — 10 regions, and the AsiA protein interacts with T4 MotA protein (255) bound to consensus DNA sequences centered at —30 positions, called motA boxes (166, 251, 383) (figure 18-4). AsiA is a dimer in solution, but a monomer when associated with ct70. The dimer interface of one monomer is alternatively used to contact ct70 (230). Two sequenced T4-related phages, RB49 (87) and KVP 40 (262), apparently lack the middle mode transcription.

Collectively, prereplicative genes encode: (i) nucleases that degrade the host DNA; (ii) enzymes of the deoxyribonu-cleotide biosynthesis complex; (iii) proteins of the replication and recombination machines; (iv) proteins that modify the T4 DNA to protect it from degradation by its own nucleases and from other restriction enzymes; (v) several tRNAs, which are processed from precursor RNAs and supplement host tRNAs during translation; (vi) proteins that modify structure and function of the host RNA polymerase; (vii) at least two RNases: RegB protein that selectively destroys certain early and probably host transcripts, and RnaseH, that degrades RNA primers (307); (viii) a differential modulator of late RNA degradation (Dmd) (184-186); and (ix) a tran-slational repressor (RegA protein) (261, 429). In addition, some prereplicative transcripts serve as primers for leadingstrand DNA synthesis in origins of replication (see below).

0 0

Post a comment