Abstract
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.