Evidence for high affinity binding-protein dependent transport systems in gram-positive bacteria and in Mycoplasma.

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Abstract
Gram‐negative bacteria are surrounded by two membranes. In these bacteria, a class of high affinity transport systems for concentrating substrates from the medium into the cell, involves a binding protein located between the outer and inner membranes, in the periplasmic region. These ‘periplasmic binding‐proteins’ are thought to bind the substrate in the vicinity of the inner membrane, and to transfer it to a complex of inner membrane proteins for concentration into the cytoplasm. We report evidence leading us to propose that a Gram‐positive bacterium, Streptococcus pneumoniae, and a mycoplasma, Mycoplasma hyorhinis, which are surrounded by a single membrane and have therefore no periplasmic region, possess an equivalent to the high affinity periplasmic binding‐protein dependent transport systems, i.e. extra‐cytoplasmic binding lipoprotein dependent transport systems. The ‘binding lipoproteins’ would be maintained at proximity of the inner membrane by insertion of their N‐terminal glyceride‐cysteine into this membrane.