Secretory proteins function as agents for numerous cell-cell interactions and determine the survival strategies adopted by organisms. Using the SOSUI system for membrane proteins (Hirokawa et al., Bioinformatics, 1998) and SOSUIsignal for signal peptides (Gomi et al., CBIJ, 2004), we undertook predictive analyses of secretory proteins from 248 prokaryota using all of the amino acid sequences coded by their respective genomes. The number of secretory proteins exhibited a strong positive

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... on with the number of total open reading frames, with analysis of these correlations revealing that prokaryotic organisms could be placed into several groups. Symbiotic or obligate parasitic organisms in eukaryotic cells with less than 1200 open reading frames exhibited a single linear relationship between the number of secretory proteins and the total number of open reading frames. Conversely, free-living organisms with more than 2500 open reading frames could be grouped into three linear relationships. The intercept with the axis of the number of open reading frames in the linear relationships was approximately 300 genes for the survival of symbiotic or obligate parasitic organisms and approximately 700 for the free-living organisms. The factor responsible for distinguishing between the different categories of organisms appeared to be G+C content and the number of open reading frames. The roles of secretory proteins and membrane proteins were discussed on the basis of the ratios of those proteins. The list of all predicted secretory proteins for 248 prokaryota is available through the internet at the URL: http://bp.nuap.nagoya-u.ac.jp/sosui/sosuisignal/SOSUIsignalDB/.