LITMUS: multipurpose cloning vectors with a novel system for bidirectional in vitro transcription.

We describe the construction and uses of a set of four multipurpose cloning vectors: LITMUS 28, 29, 38 and 39. The vectors feature the high-copy pUC origin and an M13 origin for single-stranded DNA production as well as polylinker sites for most commercially available restriction enzymes that recognize nondegenerate hexanucleotide sites and yield 4-base sticky ends upon cleavage. Sites are arranged, without overlaps, to permit linker addition to blunt-ended fragments and unidirectional nested deletions and are within the lacZ alpha gene to facilitate blue-white screening. Finally, the polylinkers are flanked by a pair of opposing modified T7 promoters to allow in vitro transcription of either strand of a cloned insert with T7 RNA polymerase. Selective unidirectional transcription from one promoter is achieved by cleaving the other at an internal restriction site (AflII or SpeI). Both modified promoters are fully active under standard RNA probe synthesis conditions. In Southern blots of Dirofilaria immitis genomic DNA, an RNA probe prepared from LITMUS performed equivalently to the same RNA probe made from a wild-type promoter vector and a DNA probe prepared by random priming.