Bacterial genome size reduction by experimental evolution.

Bacterial evolution toward endosymbiosis with eukaryotic cells is associated with extensive bacterial genome reduction and loss of metabolic and regulatory capabilities. Here we examined the rate and process of genome reduction in the bacterium Salmonella enterica by a serial passage experimental evolution procedure. The initial rate of DNA loss was estimated to be 0.05 bp per chromosome per generation for a WT bacterium and approximately 50-fold higher for a mutS mutant defective in methyl-directed DNA mismatch repair. The endpoints were identified for seven chromosomal deletions isolated during serial passage and in two separate genetic selections. Deletions ranged in size from 1 to 202 kb, and most of them were not associated with DNA repeats, indicating that they were formed via RecA-independent recombination events. These results suggest that extensive genome reduction can occur on a short evolutionary time scale and that RecA-dependent homologous recombination only plays a limited role in this process of jettisoning superfluous DNA.

Myofibrillar M-band structure and composition of physiologically defined rat motor units.

The isometric contraction time of 19 fast and slow rat motor units in the soleus and the anterior tibial muscles were recorded. The motor unit fibers, subsequently distinguished by glycogen depletion, were histochemically differentiated into fiber types and analyzed immunohistochemically for high molecular weight M-band proteins, as well as ultrastructurally for M-band fine structure, Z-disc width, and volume density of mitochondria. All fibers belonging to slow-twitch motor units in both the anterior tibialis and soleus muscles were histochemically classed as type 1. They lacked the Mr 165,000 M-protein, showed ultrastructurally a four-line M-band pattern, and had broad Z-discs, whereas the volume density of the mitochondria varied considerably. Muscle fibers belonging to the fast-twitch motor units were histochemically classed as types 2A and 2B in anterior tibialis and type 2A in soleus. They contained a three- or a five-line M-band pattern and medium-to-thin Z-discs in the anterior tibialis and a five-line M-band pattern and broad Z-discs in the soleus. Furthermore, the volume density of mitochondria showed considerable variation within and in between soleus and anterior tibialis type 2 fibers. As the differences in M-band composition and structure between fiber types overrode the intragroup variability in contraction times of slow and fast units within and between the two muscles, it is concluded that the M-band composition and structure is fundamentally related to whether the fiber is innervated by a slow or fast motor neuron, whereas other parameters such as contraction time, Z-disc width, and mitochondrial content of fibers of fast and slow units are relative and vary between muscles. Thus the M-band appearance can be used as a reliable marker to distinguish between fibers of slow- and fast-twitch motor units in rat leg muscles.

Contraction time, histochemical type, and terminal cisternae volume of rat motor units.

Isometric contraction time (Tc) of 19 fast and slow rat motor units in the soleus and the tibialis anterior muscles was measured. The motor unit fibers subsequently marked by glycogen depletion were histochemically fiber typed as well as analyzed ultrastructurally with respect to volume of the sarcoplasmic reticulum terminal cisternae. The volume density of terminal cisternae (VVTC) was inversely related to Tc over the whole range of motor units, irrespective of type of muscle. The continuous variations in VVTC are concluded to match the extrinsic control of time course of contraction, and fusion frequency of the motor unit to match the frequency characteristics of the individual motoneurons. The volume density dependence of terminal cisternae function would result in an indirect coupling between amount and rate of calcium release and rate of calcium recaptured. VVTC and Tc were the same for some tibialis anterior and soleus motor units in spite of the different types of myosin, indicating that the type 1 and type 2 myosins have specific structural differences in fast as compared with slow muscles.

Transmission and contraction fatigue of rat motor units in relation to succinate dehydrogenase activity of motor unit fibres.

1. The fatigue in rat anterior tibial (a.t.) motor units was studied and related to microphotometric determinations of succinate dehydrogenase (SDH) activity of the motor unit muscle fibres. 2. Anterior tibial contains fast-twitch type II fibre units with an average contraction time of 11 msec and about 5% slow-twitch type I fibre units with an average contraction time of 20 msec. 3. In type II fibres stained for SDH, absorbance varied continuously from 0.046 to 0.569 and inversely to fibre size, except for the largest fibres. 4. Resistance to fatigue of fast motor units to 100 Hz intermittent stimulation varied continuously within a wide range in near linear relations to absorbance for SDH of unit fibres and inversely to tetanic tension, except for motor units with the largest fibres and the largest tetanic tension. 5. Neither resistance to fatigue nor SDH activity lent itself to any categorization of motor units or fibres into well demarcated functional or histochemical types, since both parameters varied continuously in the unit and fibre population of the muscle. 6. The direct relation between resistance to fatigue of fast-twitch motor units and SDH activity of unit fibres appeared valid for fatigue resistance of: (a) neuromuscular transmission, tested with 100 Hz intermittent stimulation which gave concomitant failure of electrical and mechanical response, (b) excitation--contraction coupling, demonstrated by post-stimulatory depression of twitch tension with preserved maximum tetanus tension and action potential, and (c) contractile mechanism; excitation--contraction coupling?, tested with low frequency stimulation which gave decline of twitch and maximum tetanus tension with preserved action potential. 7. It is suggested that the endurance of each link in the chain of events leading to contraction, including neuromuscular junction and the excitation--contraction coupling system, is under aerobic conditions matched to the contractile capacity of the fibre expressed by its oxidative enzyme activity.

The motor unit: anatomy and histochemical functional correlations.

The glycogen depletion technique has demonstrated the distribution of fibres in normal and reinnervated motor units, the histochemical uniformity of its fibres and enabled direct correlation between histochemistry and function. Homogeneity exhibits the capacity of the motoneurone to determine histochemical and related functional properties of its muscle fibres at a specific level. Changes in rat soleus motor units during growth indicate mechanisms by which the motoneurone is able to alter this level even qualitatively in order to adapt the muscle to its usage.

Adaptive transformation of rat soleus motor units during growth.

Soleus motor units of 5 and 34 week-old rats, weighing 150 and 520 g respectively, were examined for contraction time at 36 degrees C, innervation ratio as well as cross-sectional area, ATPase and SDH activity of fibres. The muscle was continuously adjusted to the growth of the animal. Between 5 and 34 weeks the proportion of Type II fibre units with 15-26 msec contraction time decreased from 33% to 10% and Type I fibre units with 27-40 msec contraction time thus increased from 67 to 90%. This was parallelled by approximately the same relative changes in number of Type II and Type I fibres.

Mapping of motor units in experimentally reinnervated rat muscle. Interpretation of histochemical and atrophic fibre patterns in neurogenic lesions.

The distribution of fibres in the motor units of reinnervated muscle is demonstrated. As shown in some experimental models, the histochemical and atrophic fibre patterns in neurogenic lesions are determined by the morphology of the specific motor units involved. This also helps to explain the alterations of motor unit action currents in neurogenic lesions.