This section explains how to use the program screen_seq to filter out unwanted readings: i.e. how to search for and separate readings containing the sequences of extraneous DNA, such as vector or bacterial sequences. We have separated this task from that of locating and marking the extents of sequencing vector and other cloning vectors. There we require precise identification of the junction between the vectors and the target DNA. The filtering process described here is designed to spot strong matches between readings and a panel of possible contaminating sequences, and it splits readings into passes and fails. Readings that fail have a PS line containing the word "contaminant" added to their experiment file.
Normal usage would be to compare a batch of readings in experiment file format against a batch of possible contaminant sequences stored in (at present) simple text files. Each batch is presented to the program as a file of file names, and the program will write out two new files of file names: one containing the names of the files that do not match any of the contaminant sequences (the passes), and the other those that do match (the fails). It is also possible to compare single readings and single contaminant files by giving their file names (i.e. it is not necessary to use a file of file names for single files).
Given the frequent need to compare against the full E. coli genome the algorithm is designed to be fast. Only one parameter is required: the minimum match length, min_match. All readings which contain a segment of sequence of length min_match which exactly matches a possible contaminant sequence are filtered out.
The search is conducted only over the clipped portion of the readings. On our aging Alpha machine it takes about 1 second to compare both strands of a reading against the 4.7 million bases of E. coli.