GC-rich intra-operonic spacers in prokaryotes: Possible relation to gene order conservation

Genome-scale compositional analyses of non-coding sequences from 410 microbes of varying GC-content, lineage, environment/life-style, reveal presence of a distinct trend in GC-usage in spacers between intra-operonic and extra-operonic gene-pairs. For most of the microbes, average GC-content of the intra-operonic spacers are consistently higher than those between extra-operonic unidirectional gene-pairs. Also, unidirectional gene-pairs exhibiting higher cross-species conservation, irrespective of their operonic context, house relatively GC-rich spacers. A few prokaryotes, most of which represent known cases of genome degradation, stand out as exceptions defying this trend. GC-enrichment of intra-operonic spacers therefore appears to be an evolutionary strategy facilitating preservation of operonic gene-order.