Sanchez, Joaquin (1);Jose Marco, V. (2) - Bilateral inverse symmetry in whole bacterial chromosomes

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Title: Bilateral inverse symmetry in whole bacterial chromosomes	P139
Sanchez, Joaquin (1); Jose Marco, V. (2) joaquin.sanchez@microbio.gu.se (1) Department of Medical Microbiology and Immunology, University of Gothenburg, Gothenburg, SWEDEN, SE413-46 and Facultad de Medicina, UAEM, Av. Universidad 1001, Cuernavaca, Morelos, CP62210, MÉXICO (2) Instituto de Investigaciones Biomédicas, UNAM, Ciudad Universitaria, México, D.F., CP70228, MÉXICO.

We demonstrate correlations in the frequencies of complementary triplets between bacterial chromosomal halves (thereby designated bilateral inverse symmetry or BIS). BIS was found to express at the lower levels including single nucleotides and we propose that this explains why Chargaff's second parity rule is followed by whole bacterial chromosomes. However, using ad hoc shuffled sequences we show that single-nucleotide BIS is not sufficient to account for BIS at higher-order levels. This implies that BIS is not simply a reflection of base composition differences along the chromosome. One way BIS could have been generated is through well-studied strand biased-mutation and/or selection mechanisms. Nonetheless, we discuss the very attractive alternative hypothesis that BIS may be the remnant of an ancient whole-genome inverse duplication event. Such model would fully agree with strand compositional asymmetry, with the position of GC-skews, and with bi-directional replication of the chromosome.