A component of the side tail fiber of Escherichia coli bacteriophage lambda can functionally replace the receptor-recognizing part of a long tail fiber protein of the unrelated bacteriophage T4.

The distal part of the long tail fiber of Escherichia coli bacteriophage T4 consists of a dimer of protein 37. Dimerization requires the catalytic action of protein 38, which is encoded by T4 and is not present in the virion. It had previously been shown that gene tfa of the otherwise entirely unrelated phage lambda can functionally replace gene 38. Open reading frame (ORF) 314, which encodes a protein that exhibits homology to a COOH-terminal area of protein 37, is located immediately upstream of tfa. The gene was cloned and expressed in E. coli. An antiserum against the corresponding polypeptide showed that it was present in phage lambda. The serum also reacted with the long tail fibers of phage T4 near their free ends. An area of the gene encoding a COOH-terminal region of ORF 314 was recombined, together with tfa, into the genome of T4, thus replacing gene 38 and a part of gene 37 that codes for a COOH-terminal part of protein 37. Such T4-lambda hybrids, unlike T4, required the presence of outer membrane protein OmpC for infection of E. coli B. An ompC missense mutant of E. coli K-12, which was still sensitive to T4, was resistant to these hybrids. We conclude that the ORF 314 protein represents a subunit of the side tail fibers of phage lambda which probably recognize the OmpC protein. ORF 314 was designated stf (side tail fiber). The results also offer an explanation for the very unusual fact that, despite identical genomic organizations, T4 and T2 produce totally different proteins 38. An ancestor of T4 from the T2 lineage may have picked up tfa and stf from a lambdoid phase, thus possibly demonstrating horizontal gene transfer between unrelated phage species.