misha (version 5.4.0)

gsynth.replace_kmer: Iteratively replace a k-mer in the genome

Description

Performs an iterative replacement of a target k-mer with a replacement sequence. This is useful for creating synthetic genomes with specific motifs removed (e.g., creating a CpG-null genome by iteratively swapping CG to GC).

Usage

gsynth.replace_kmer(
  target,
  replacement,
  output_path = NULL,
  output_format = c("misha", "fasta", "vector"),
  intervals = NULL,
  check_composition = TRUE
)

Value

When output_format is "misha" or "fasta", returns invisible NULL and writes to output_path. When output_format is "vector", returns a character vector of modified sequences.

Arguments

target

The k-mer sequence to remove (e.g., "CG").

replacement

The replacement sequence (e.g., "GC").

output_path

Path to the output file (ignored when output_format = "vector").

output_format

Output format:

• "misha": .seq binary format (default)

• "fasta": FASTA text format

• "vector": Return sequences as a character vector (does not write to file)

intervals

Genomic intervals to process. If NULL, uses all chromosomes.

check_composition

Logical. If TRUE (default), ensures target and replacement have the same nucleotide composition (preserving exact base counts).

Details

Bubble Sort / Iterative Logic: The function scans the sequence and replaces occurrences of target with replacement. If a replacement creates a new instance of target (e.g., removing "CG" with "GC" in the sequence "CCG" -> "CGC"), the new instance is also replaced. This continues until the sequence is free of the target k-mer.

When target and replacement are permutations of each other (e.g., "CG" and "GC"), this acts as a "bubble sort" of nucleotides, moving bases locally without altering the total GC content or base counts of the genome.

Examples

if (FALSE) {
# Robust removal of all CpG dinucleotides (preserving GC%)
gsynth.replace_kmer(
    target = "CG",
    replacement = "GC",
    output_path = "genome_no_cpg.seq",
    output_format = "misha"
)
}