Relative influence of natural selection and mutational pressure on the genes of primary and secondary chromosome of multichromosomal bacteria

Abstract

Rhodobacter sphaeroides is the first reported multiple chromosomal bacteria, a rod-shaped, Gram-negative bacterium [1]. The researchers provide a complete physical map of the Rhodobacter sphaeroides genome by obtaining chromosomal DNA fragments through restriction digestion with endonuclease from the genomic DNA, which support in establishing the existence of more than one chromosome [1]. Chromosomes may produce by three different mechanisms: by the separation of a single chromosome, by chromosome copying, or by integration of a large plasmid with essential genes [2]. Of these processes, the theory of plasmid derived chromosome is more widely acceptable and has the greatest support because origins of replication of some secondary chromosomes have similarity with the plasmids oriC [2]. The emerging picture is that Multichromosomal bacteria have one primary chromosome (chromosome1) carrying most housekeeping genes, and generally one or two secondary chromosomes having some plasmid features which is recognisable and also contains some essential genes [3]. Under a certain environmental (external) conditions specified amplification of one chromosome can alter gene pool and thereby the levels of gene expression [3].