Direction of transcription in bacteriophage T5 first-step transfer DNA

Pre-early functions of bacteriophage T5 and its relatives

Coliphage T5 injects its DNA in 2 steps: the first step transfer (FST) region 7.9% is injected and its genes are expressed and only then does the remainder (second step transfer, SST) of its DNA enter the cell. In the FST region, only 2 essential genes (A1 and A2) have been identified and a third (dmp) non-essential gene codes for a deoxyribonucleotide 5' monophosphatase. Thirteen additional putative ORFs are present in the FST region. Numerous properties have been attributed to FST region, including SST, host DNA degradation, inhibition of host RNA and protein synthesis, restriction insensitivity and protection of T5 DNA. These effects do not occur following infection with an A1 mutant. The A2 gene seems only to be involved in SST transfer. This is puzzling since there are more seemingly unrelated effects than there are essential genes to accomplish them and it is possible that some important genes were not identified. This review attempts to analyze these problems that were first identified in the 1970-80 s. In particular, an attempt is made to determine which potential ORFs are conserved in evolution (and thus likely to be important); by comparing T5 to 10 newly isolated and completely sequenced T5-like phages. A similar approach is used to identify conserved repeats, inverted repeats and palindromes that occur in all T5-like phages in the region containing the injection stop signal (iss) and the terminase substrate. Finally, an attempt is made to re-analyze the mechanism whereby T5 protects itself from the enzymes that degrade host DNA, from the RecBCD nuclease and from restriction enzymes. For all of these FST effects new hypotheses and possible new genetic and biochemical approaches are envisaged.

The first-step transfer-DNA injection-stop signal of bacteriophage T5

Bacteriophage T5 is different from most phages in that its DNA is injected in two steps during infection. The region containing the injection stop signal (iss) has been cloned and sequenced and found to contain numerous large repeats and inverted repeats which may be part of the iss. The most impressive of these are the 31-bp repeat units (rb) which are present three times in 99 bp. The rb repeats, themselves, contain inverted repeats so that mutually exclusive stem-and-loop structures may potentially form, not only within the repeats, but also between them. Another pair of repeats (21 bp each) contains two sequences resembling DnaA protein-binding sites. The region sequenced also contains one of the T5 site-specific strand interruptions and this was found to lie at the base of a perfect 9-bp palindrome.

Cell-free transcription and translation of isolated restriction fragments localize bacteriophage T5 pre-early genes

In vitro transcription and translation of isolated restriction fragments containing all or part of the terminal redundancies of bacteriophage T5 localized virtually every pre-early gene to a small 6.3-kilobase BglI fragment. Among these genes were those encoding the A1 and A2 proteins, which are responsible for complete entry of the viral genome into its host, and the deoxynucleoside 5'-monophosphatase. A 3.9-kilobase BglI fragment containing the remainder of the pre-early region induced no proteins under these same conditions. Proteins induced by fragments including the right and left terminal redundancies were also compared and found to be identical. DNA immediately flanking the pre-early regions induced few proteins in vitro. Thus, this technique has allowed the overall gene organization of the pre-early region of T5 to be described.