An improved filamentous helper phage for generating single-stranded plasmid DNA.

Gene cloning in plasmid vectors that contain a filamentous phage intergenic region presents several advantages. However, technical difficulties have been a problem, primarily low yields of packaged single stranded (ss) plasmid DNA from the rapid, small scale procedures usually employed, and ambiguities in sequencing reactions attributed to the contamination by helper phage ss DNA. We report here the construction and some properties of a new f1 helper phage. Using this phage, R408, plasmid ss DNA is packaged and exported preferentially over phage ss DNA, and the absolute yield of plasmid ss DNA is usually increased.